Understanding the Science of Global Warming

Studies have tried to put a number on how much of global warming is caused by humans, and the rough answer is, all of it 

- John Cook, climate communication fellow, Global Change Institute, University of Queensland

Anthropogenic global warming affects every human being on the planet and what we as a society do in the next few years will affect all life on earth and all future generations of humanity. Every single global citizens opinion and action will count but in order to make effective decisions in such a complex and politically loaded issue, citizens need to empower themselves with education of a complex subject. The ubiquity of climate denialism adds confusion and misconception to this already difficult-to-understand area. This page attempts to provide a clearly understandable narrative into this complex subject that can help the reader clear up confusion and misconception created by the climate denial movement. An overview of the important concept of net energy balance is first given by James Hansen. Then we establish the natural carbon cycle and then shows how human activity disturbs the carbon cycle, leading to anthropogenic global warming.

The simplest explanation of the global warming problem is shown by the graphic below.

nasa ceres energy balance

Figure 0: Earth net energy balance (Source: NASA Clouds and Earth’s Radiant Energy System Ceres) )

The Earth is a closed system. Radiant solar energy enters the system as long wave (LW) radiation and is reflected back out into space as short wave radiation (SW). To remain at zero net energy, the amount that enters must equal the amount that leaves. If course that energy sticks around for awhile to make life possible but it eventually radiates back out into space. If more energy leaves than enters, the net energy amount becomes negative, and the planet gets colder. On the other hand, if it is positive, it means there is more energy entering than leaving and that heats up the planet.  Up until recent times, there has been a net zero balance maintained. With the beginning of the Industrial Revoluion, however, the emission of anthropogenic (man-made) greenhouse gases (GHG) have created an extra layer of insulation in the atmosphere, preventing LW radiation from fully escaping the earth system and upsetting the natural balance maintained by naturally occurring greenhouse gases. How much energy are we talking about?…

The first thing to realize is that global warming usually means anthropogenic global warming – that is, warming created by humans. The greenhouse effect, when kept in balance is fundamental to maintain the health of ecosystems on the planet. Without this layer of gas which traps heat in the thin layer of the atmosphere where much of life makes its home, our global mean air temperature would be a balmy – 18 Deg C (NASA 2013). However, if we disturb this layer by increasing too many greenhouse gases, we begin to trap more energy in the closed earth system and that leads to impacts detrimental to healthy ecosystems. Currently, due to anthropogenic emissions, we are adding the equivalent of four Hiroshima bomb worth of energy to the earth system every second (Cook 2013). This net energy balance is established by net energy balance measurements made by NASA. Four Hiroshima bombs a second translates into 345,600 Hiroshima bombs each DAY or 126,144,000 Hiroshima bombs every year. That’s a LOT of energy. Climate denialists have a hard time explaining how 126 million Hiroshima bombs worth of energy have no effect on the earth’s climate system. What does that kind of scale of energy do to our global climate system?

The widget to the left measures the net heat energy measured in units of Hiroshima bombs that anthropogenic emissions are causing because the increase in astmospheric CO2 traps it in, disallowing it to radiate into space. The total heat content data used in the widget is based on Nuccitelli et al (2012), which in turn is based on data from Church et al (2011), and the ocean temperatures above 2000 meters from Levitus et al (2012).



Emissions today are equivalent to 4 Hiroshima bombs worth of energy entering the Earth system every second

(Source: John Cook, University of Queensland)

The Earth’s Energy Balance

James Hansen gives an excellent explanation of the net energy balance below.

Earth’s Energy Imbalance

Deployment of an international array of Argo floats, measuring ocean heat content to a depth of 2000 m, was completed during the past decade, allowing the best assessment so far of Earth’s energy imbalance. The observed planetary energy gain during the recent strong solar minimum reveals that the solar forcing of climate, although significant, is overwhelmed by a much larger net human-made climate forcing. The measured imbalance confirms that, if other climate forcings are fixed, atmospheric CO2 must be reduced to about 350 ppm or less to stop global warming. In our recently published paper (Hansen et al., 2011), we also show that climate forcing by human-made aerosols (fine particles in the air) is larger than usually assumed, implying an urgent need for accurate global aerosol measurements to help interpret continuing climate change.

Earth’s energy imbalance is the difference between the amount of solar energy absorbed by Earth and the amount of energy the planet radiates to space as heat. If the imbalance is positive, more energy coming in than going out, we can expect Earth to become warmer in the future — but cooler if the imbalance is negative. Earth’s energy imbalance is thus the single most crucial measure of the status of Earth’s climate and it defines expectations for future climate change.

Energy imbalance arises because of changes of the climate forcings acting on the planet in combination with the planet’s thermal inertia. For example, if the Sun becomes brighter, that is a positive forcing that will cause warming. If Earth were like Mercury, a body composed of low conductivity material and without oceans, its surface temperature would rise quickly to a level at which the planet was again radiating as much heat energy to space as the absorbed solar energy.

Earth’s temperature does not adjust as fast as Mercury’s due to the ocean’s thermal inertia, which is substantial because the ocean is mixed to considerable depths by winds and convection. Thus it requires centuries for Earth’s surface temperature to respond fully to a climate forcing.

Climate forcings are imposed perturbations to Earth’s energy balance. Natural forcings include change of the Sun’s brightness and volcanic eruptions that deposit aerosols in the stratosphere, thus cooling Earth by reflecting sunlight back to space. Principal human-made climate forcings are greenhouse gases (mainly CO2), which cause warming by trapping Earth’s heat radiation, and human-made aerosols, which, like volcanic aerosols, reflect sunlight and have a cooling effect.

NASA GISS earth energy imbalance fig1_s

Figure 1. Contributions to Earth’s (positive) energy imbalance in 2005-2010. Estimates for the deep Southern and Abyssal Oceans are by Purkey and Johnson (2010) based on sparse observations. (Source: NASA/GISS)

Let’s consider the effect of a long-lived climate forcing. Say the Sun becomes brighter, staying brighter for a century or longer, or humans increase long-lived greenhouse gases. Either forcing results in more energy coming in than going out. As the planet warms in response to this imbalance, the heat radiated to space by Earth increases. Eventually Earth will reach a global temperature warm enough to radiate to space as much energy as it receives from the Sun, thus stabilizing climate at the new level. At any time during this process the remaining planetary energy imbalance allows us to estimate how much global warming is still “in the pipeline.”

Many nations began, about a decade ago, to deploy floats around the world ocean that could “yo-yo” an instrument measuring ocean temperature to a depth of 2 km. By 2006 there were about 3000 floats covering most of the world ocean. These floats allowed von Schuckmann and Le Traon (2011) to estimate that during the 6-year period 2005-2010 the upper 2 km of the world ocean gained energy at a rate 0.41 W/m2 averaged over the planet.

We used other measurements to estimate the energy going into the deeper ocean, into the continents, and into melting of ice worldwide in the period 2005-2010. We found a total Earth energy imbalance of +0.58±0.15 W/m2 divided as shown in Fig. 1.

The role of the Sun

Plot of solar irradiance from 1975 to 2010
Figure 2: Solar irradiance in the era of accurate satellite data. Left scale is the energy passing through an area perpendicular to Sun-Earth line. Averaged over Earth’s surface the absorbed solar energy is ~240 W/m2, so the amplitude of solar variability is a forcing of ~0.25 W/m2. (Source: NASA/GISS)
The measured positive imbalance in 2005-2010 is particularly important because it occurred during the deepest solar minimum in the period of accurate solar monitoring (Fig. 1). If the Sun were the only climate forcing or the dominant climate forcing, then the planet would gain energy during the solar maxima, but lose energy during solar minima.

The fact that Earth gained energy at a rate 0.58 W/m2 during a deep prolonged solar minimum reveals that there is a strong positive forcing overwhelming the negative forcing by below-average solar irradiance. That result is not a surprise, given knowledge of other forcings, but it provides unequivocal refutation of assertions that the Sun is the dominant climate forcing.

Target CO2

The measured planetary energy imbalance provides an immediate accurate assessment of how much atmospheric CO2would need to be reduced to restore Earth’s energy balance, which is the basic requirement for stabilizing climate. If other climate forcings were unchanged, increasing Earth’s radiation to space by 0.5 W/m2 would require reducing CO2 by ~30 ppm to 360 ppm. However, given that the imbalance of 0.58±0.15 W/m2 was measured during a deep solar minimum, it is probably necessary to increase radiation to space by closer to 0.75 W/m2, which would require reducing CO2 to ~345 ppm, other forcings being unchanged. Thus the Earth’s energy imbalance confirms an earlier estimate on other grounds that CO2 must be reduced to about 350 ppm or less to stabilize climate (Hansen et al., 2008).


The measured planetary energy imbalance also allows us to estimate the climate forcing caused by human-made atmospheric aerosols. This is important because the aerosol forcing is believed to be large, but it is practically unmeasured.

NASA GISS earth energy imbalance fig3_s

Figure 3. Schematic diagram of human-made climate forcings by greenhouse gases, aerosols, and their net effect. (Source: NASA/GISS)
+ View larger image or PDF

The human-made greenhouse gas (GHG) forcing is known to be about +3 W/m2 (Fig. 3). The net human-made aerosol forcing is negative (cooling), but its magnitude is uncertain within a broad range (Fig. 3). The aerosol forcing is complex because there are several aerosol types, with some aerosols, such as black soot, partially absorbing incident sunlight, thus heating the atmosphere. Also aerosols serve as condensation nuclei for water vapor, thus causing additional aerosol climate forcing by altering cloud properties. As a result, sophisticated global measurements are needed to define the aerosol climate forcing, as discussed below.

The importance of knowing the aerosol forcing is shown by considering the following two cases: (1) aerosol forcing about -1 W/m2, such that the net climate forcing is ~ 2 W/m2, (2) aerosol forcing of -2 W/m2, yielding a net forcing ~1 W/m2. Both cases are possible, because of the uncertainty in the aerosol forcing.

Which alternative is closer to the truth defines the terms of a “Faustian bargain” that humanity has set for itself. Global warming so far has been limited, as aerosol cooling has partially offset greenhouse gas warming. But aerosols remain airborne only several days, so they must be pumped into the air faster and faster to keep pace with increasing long-lived greenhouse gases (much of the CO2 from fossil fuel emissions will remain in the air for several millennia). However, concern about health effects of particulate air pollution is likely to lead to eventual reduction of human-made aerosols. Thereupon humanity’s Faustian payment will come due.

If the true net forcing is +2 W/m2 (aerosol forcing -1 W/m2), even a major effort to clean up aerosols, say reduction by half, increases the net forcing only 25% (from 2 W/m2 to 2.5 W/m2). But if the net forcing is +1 W/m2 (aerosol forcing -2 W/m2), reducing aerosols by half doubles the net climate forcing (from 1 W/m2 to 2 W/m2). Given that global climate effects are already observed (IPCC, 2007; Hansen et al., 2012), doubling the climate forcing suggests that humanity may face a grievous Faustian payment.

NASA GISS earth energy imbalance fig4_s

Figure 4. Expected Earth energy imbalance for three choices of aerosol climate forcing. Measured imbalance, close to 0.6 W/m2, implies that aerosol forcing is close to -1.6 W/m2. (Source: NASA/GISS)

Most climate models contributing to the last assessment by the Intergovernmental Panel on Climate Change (IPCC, 2007) employed aerosol forcings in the range -0.5 to -1.1 W/m2 and achieved good agreement with observed global warming over the past century, suggesting that the aerosol forcing is only moderate. However, there is an ambiguity in the climate models. Most of the models used in IPCC (2007) mix heat efficiently into the intermediate and deep ocean, resulting in the need for a large climate forcing (~2 W/m2) to warm Earth’s surface by the observed 0.8°C over the past century. But if the ocean mixes heat into the deeper ocean less efficiently, the net climate forcing needed to match observed global warming is smaller.

Earth’s energy imbalance, if measured accurately, provides one way to resolve this ambiguity. The case with rapid ocean mixing and small aerosol forcing requires a large planetary energy imbalance to yield the observed surface warming. The planetary energy imbalance required to yield the observed warming for different choices of aerosol optical depth is shown in Fig. 4, based on a simplified representation of global climate simulations (Hansen et al., 2011).

Measured Earth energy imbalance, +0.58 W/m2 during 2005-2010, implies that the aerosol forcing is about -1.6 W/m2, a greater negative forcing than employed in most IPCC models. We discuss multiple lines of evidence that most climate models employed in these earlier studies had moderately excessive ocean mixing, which could account for the fact that they achieved a good fit to observed global temperature change with a smaller aerosol forcing.

The large (negative) aerosol climate forcing makes it imperative that we achieve a better understanding of the aerosols that cause this forcing. Unfortunately, the first satellite capable of measuring detailed aerosol physical properties, the Glory mission (Mishchenko et al., 2007), suffered a launch failure. It is urgent that a replacement mission be carried out, as the present net effect of changing emissions in developing and developed countries is highly uncertain

Global measurements to assess the aerosol indirect climate forcing, via aerosol effects on clouds, require simultaneous high precision polarimetric measurements of reflected solar radiation and interferometric measurements of emitted heat radiation with the two instruments looking at the same area at the same time. Such a mission concept has been defined (Hansen et al., 1993) and recent reassessments indicate that it could be achieved at a cost of about $100M if carried out by the private sector without a requirement for undue government review panels.

(Source: NASA GISS, James Hansen et al., 2012)

The Confusion of Global Warming and Surface Air Temperatures (SAT)

Often, global warming is mistaken for Surface Air Temperatures (SAT). A favorite trick of climate denialists is to show the most recent Surface Air Temperature warming hiatus that began around 1998 and continues on to today as contradiction to global warming. The two are not the same. Global warming is concerned about the net energy balance. The energy entering the earth system is the solar radiation and the energy which leaves is the reflected infrared. While that energy is here in the earth system, it is stored in various forms. Only approximately 2% of the solar radiation energy input entering the earth climate system is stored in the atmosphere. Most of it is stored in the oceans.



global heat accumulation graph Nuccitelli_OHC_450


Figure 5: a) Heat distribution on planet (Source: IPCC c/o Skeptical Science), b) Heat distribution in ocean and land (Source: Nuccitelli et al. 2012)

An excellent piece written in the June 2013 online edition of Guardian by researcher Dana Nuccitelli called We haven’t hit the global warming pause button explains the difference between Global Warming and Surface Air Temperature:

We haven’t hit the global warming pause button

Only about 2 percent of the planet’s overall warming heats the atmosphere, so if we focus only on surface air temperatures, we miss 98 percent of the overall warming of the globe. About 90 percent of the warming of the planet is absorbed in heating the oceans. However, until the past few years, our measurements of ocean temperatures (especially of the deep oceans) were somewhat lacking. Our measurements of surface air temperatures were much more accurate, and so when people spoke of “global warming,” they tended to focus on air temperatures.

In the 1980s and 1990s when air temperatures were warming in step with the overall warming of the planet, that was fine. However, over the past decade, the warming of surface air temperatures has slowed. At the same time, the overall warming of the planet has continued, and if anything it has accelerated. This has been difficult to reconcile for those who previously focused on surface air temperatures – what do we say about “global warming” now?

The result is a spate of articles from the New York TimesWashington PostThe New Republic, and Der Spiegel, all of which get most of the facts right (including noting the warming of the oceans), but that all begin from the premise that “global warming” has slowed. It would be more accurate to say that global surface air warming has slowed, but the overall warming of the Earth’s climate has sped up.

This is the conclusion of several papers published in the past year, including studies led by Sydney Levitus of the National Oceanic and Atmospheric Administration (NOAA), Magdalena Balmaseda of the European Centre for Medium Range Weather Forecast, Virginie Guemasof the Catalán Institute of Climate Science, and myself. When the warming of the Earth’s entire climate system is considered, global warming continues to rise at a rate equivalent to about 4 Hiroshima atomic bomb detonations per second, faster over the past 15 years than the prior 15 years.

The small fraction of that warming that’s expressed by changes in surface air temperature does appear to have slowed over the past decade. Research by Masahiro Watanabe of the Japanese Atmosphere and Ocean Research Institute suggests this is mainly due to more efficient transfer of heat to the deep oceans. Consistent with model simulations led by NOAA’s Gerald Meehl, Watanabe finds that we sometimes expect “hiatus decades” to occur, when surface air temperatures don’t warm because more heat is transferred to the deep ocean layers.

(Source: The Guardian)

The Simple Truth of CO2 as a Greenhouse Gas

A simple NOAA lab experiment illustrating thef heat trapping effect of CO2 using Alka Seltzer tablets

MIT K12 video showing a simple experiment illustrating CO2 heat trapping using baking soda and vineagar

The hypothesis that CO2 is a greenhouse gas responsible for warming the atmosphere was first proposed in 1896 by Swedish scientist Svante Arrhenius, who later won the Nobel Prize in 1903. In Arrhenius’ paper, he discussed how variations in carbon dioxide concentration in the atmosphere must be naturally variable, and that variations in carbon dioxide are what lead to excessively warm and cold periods in the Earth’s history. Many of his assumptions have since been validated. This research paper was the product of over five decades worth of work. It was by no means straightforward and there were heated debates with other scientists who doubted his claims, which led to further  investigations of the climate effects of ice, the oceans, trees, industrial pollution and more.

Arrhenius’s work, however, remained mostly ignored until the 1950s when it became increasingly clear to scientists that carbon dioxide resulting from industrialization was going to be a problem for the climate:

  1. As early as 1965, a government committee warned President Johnson of the dangers of global warming
  2. In 1975, Wally Broecker published his paper “Are we on the brink of a pronounced global warming?” in Science

In spite of the complexity of climate science, the simple fact underlying climate change is the same. Whether it’s a 2-liter Coke bottle or the Earth’s atmosphere, adding more carbon dioxide to a mixture of gasses will cause the mixture to trap more heat. Today, we are witnessing what Arrenhius predicted in 1896. Since the beginning of the industrial revolution, carbon dioxide emissions have steadily increased, causing the global temperature to increase.

Figure 6: A) CO2 levels based upon comparison of atmospheric samples contained in ice cores and more recent direct measurements, provides evidence that atmospheric CO2 has increased since the Industrial Revolution. B) Global Annual Mean Surface Air Temperature Change

(Source: NASA & NOAA, Forbes)

The Carbon Cycle

Carbon is the fourth most abundant element in the universe and a naturally occurring element on our planet. It is a critical building block of life and there is a fixed amount of it which cycles between living (biotic) and the nonliving (abiotic) environment. As it cycles,  it changes states moving between the biosphere, geosphere, ocean and atmosphere. This biogeochemical movement is called the Carbon Cycle. Click on each graphic below to go the respective animation that demonstrates the carbon cycle.

GAO: Depiction of the Global Carbon Cycle Changes Over Time

NASA presentation of the carbon cycle

Biogeochemical Cycling: All the cycles that includes carbon, water, nitrogen and phosphorous cycling

Carbon cycle between biosphere and atmosphere

Ocean carbon cycle

Solid Earth carbon cycle


Figure 7: Click on the Carbon Cycle Game

Figure 8: University of Waikato Carbon Cycle Game (Click to go to the U of Waikato gamesite)

Geological Carbon Cycle

This cycle is driven by plate tectonics, including processes such as volcanic eruptions and burial of carbon-rich sediments on the ocean floor and happens on timescales of millions of years.

The geological component of the carbon cycle is where carbon interacts with the rock cycle in the processes of:

  • weathering and dissolution
  • precipitation of minerals
  • burial and subduction
  • volcanism

Steps in the Cycle

  1. In the atmosphere, carbon dioxide (CO2) and water react to form carbonic acid.
  2. When carbonic acid reaches the ground via rain  it reacts with minerals at the earth’s surface and slowly dissolves them into their component ions through the process of chemical weathering
  3. These component ions are carried in surface waters like streams and rivers  to the ocean, where they precipitate out as minerals like calcite (CaCO3)
  4. Through continued deposition and burial, this calcite sediment forms the rock called limestone
  5. This cycle continues as seafloor spreading pushes the seafloor under continental margins in the process of subduction
  6. As seafloor carbon is pushed deeper into the earth by tectonic forces, it heats up, eventually melts, and can rise back up to the surface, where it is released as CO2 and returned to the atmosphere
  7. This return to the atmosphere can occur violently through volcanic eruptions, or more gradually in seeps, vents, and CO2-rich hotsprings
  8. Tectonic uplift can also occur and expose previously buried limestone (ie.  Himalayas where some of the world’s highest peaks are formed of material that was once at the bottom of the ocean)
  9. Weathering, subduction, and volcanism control atmospheric carbon dioxide concentrations over time periods of hundreds of millions of years

Biological Carbon Cycle

Figure 9: Biological Carbon Cycle (Source Victorian Resources Online) 

This cycle is driven by respiration by plants and animals and photosynthesis by plants. It happens on timescale ranging from days to thousands of years. Biology plays an important role in the movement of carbon between land, ocean, and atmosphere through the processes of photosynthesis and respiration.


Sunlight plus carbon dioxide produce sugar (carbohydrates) and Oxygen. Plants take in carbon dioxide (CO2) from the atmosphere during photosynthesis in the day.

energy (sunlight) + 6CO2 + H2O  —>  C6H12O6 + 6O2


Respiration releases the energy contained in sugars for use in metabolism and changes carbohydrate “fuel” back into carbon dioxide, which is in turn released back to the atmosphere. Water and CO2 are produced as byproducts as well. Plants also respire and release  a smaller amount of COback into the atmosphere during respiration at night.

C6H12O6 (organic matter) + 6O2 —>  6CO2 + 6 H2O + energy

Animals are complimentary to plants in that they take in Oxygen and produce CO2

Carbon from photosynthesis is the dominant source of short term carbon cycling. The amount of carbon taken up by photosynthesis and released back to the atmosphere by respiration each year is about 1,000 times greater than the amount of carbon that moves through the geological cycle on an annual basis.

  • On land, the major exchange of carbon with the atmosphere results from these biological processes of photosynthesis and respiration
  • During daytime in the growing season, leaves absorb sunlight and take up carbon dioxide from the atmosphere
  • At the same time plants, animals, and soil microbes consume the carbon in organic matter and return carbon dioxide to the atmosphere
  • Photosynthesis stops at night when the sun cannot provide the driving energy for the reaction, though respiration continues
  • This kind of imbalance between these two processes is reflected in seasonal changes in the atmospheric CO2 concentrations: during winter in the northern hemisphere, photosynthesis ceases when many plants lose their leaves, but respiration continues. This condition leads to an increase in atmospheric COin the winter in the northern hemisphere

Global Carbon Stores

We can organize all the carbon on earth into five main pools, listed in order of the size of the pool:

Figure 10: Total carbon stores, natural and anthropogenic (Source: University of New Hampshire)

Lithosphere (Earth’s crust)

  • 66,000,000 to 100,000,000 gigatons
  • sedimentary rock deposits, such as limestone, dolomite, and chalk plus fossil fuels. 4,000 gigatons  of this is fossil fuel


  • 38,000 to 40,000 gigatons
  • dissolved carbon dioxide, and calcium carbonate shells in marine organisms


  • 1,500 to 1,600 gigatons


  • 578 gigaton in 1700 to about 766 gigaton in 1999
  • carbon dioxide, carbon monoxide, and methane.  It continues to increase at the rate of about 6.1 gigatons per year


  • 540 to 610 gigatons
  • all living and dead organisms not yet converted into soil organic matter

Earth Energy Budget

To understand how anthropogenic CO2 impacts the earth’s natural carbon cycle first requires understanding the how solar energy enters the earth system and how it interacts with it. The earth’s energy budget is the total amount of energy gains that enter the earth system and losses into leave it into space. This in turn requires understanding 3 other factors:

  • solar radiation spectrum
  • earth’s reflectivity
  • the atmosphere

Solar Radiation Spectrum

The earth receives energy from the sun in the form of a wide spectrum of solar radiation, whose characteristics are given below:

Figure 11: Wavelength spectrum of solar energy

Most of the radiant energy from the sun is concentrated in the visible and near-visible parts of the spectrum:

  • The narrow band of visible light, between 400 and 700 nm, represents 43% of the total radiant energy emitted
  • Wavelengths shorter than the visible account for 7 to 8% of the total, but are extremely important because of their high energy per photon. The shorter the wavelength of light, the more energy it contains
  • The remaining 49 – 50% of the radiant energy is spread over the wavelengths longer than those of visible light. These lie in the near infrared range from 700 to 1000 nm; the thermal infrared, between 5 and 20 microns; and the far infrared regions
  • Various components of earth’s atmosphere absorb ultraviolet and infrared solar radiation before it penetrates to the surface, but the atmosphere is quite transparent to visible light

Albedo, or Reflectivity

The albedo is a dimensionless reflection coefficient, derived from the Latin albedo “whiteness” (or reflected sunlight), in turn from albus “white”. It is the diffuse reflectivity or reflecting power of a surface and is defined as the ratio of reflected radiation from the surface to incident radiation upon it.

The planet’s albedo varies with time of the year and location, but 0.3 is the average figure. Hence, 30% of the incident solar energy is reflected into space, while 70% is absorbed by the Earth and reradiated as long wave radiation (infrared).  

The Atmosphere

The earth’s atmosphere is a very thin layer that covers the surface of the planet. It’s importance cannot be understated; it is what makes life possible. It is composed of a combination of:

  1. a mixture of gases
  2. water vapor
  3. aerosols

Atmospheric Gases

The mixture of gas consists of:

  1. 78% of nitrogen – N2
  2. 21% of oxygen – O2,
  3. 1% of argon,
  4. 1% of a mixture of trace gases including all the gases called greenhouse gases (GHG) such as CO2, Methane (CH4), Hydrofluorocarbons (HFC’s)
A Greenhouse Gas (GHG) is an atmospheric gas which is transparent to incoming shortwave solar radiation but is opaque to longer wavelength (infrared) radiation. Atmospheric greenhouse gases act as a blanket and trap infrared radiation  between the atmosphere and the surface of the planet.

Figure 12: Gas composition of the atmosphere

Water Vapor

Water vapor is considered a greenhouse gas and is 10 times more abundant than CO2. In fact, it is responsible for most of the natural greenhouse effect. Without water vapor, average temperatures would be up to 30 degrees Celsius lower. CO2, on the other hand, is responsible for a much smaller but still substantial amount of the natural warming effect. Climate denialist love to latch onto this fact to raise doubts amongst the naive that water vapor plays a far bigger role in warming the planet than does CO2. Yet the UN’s climate body the International Panel on Climate Change (IPCC) does not even list water vapor as a greenhouse gas. The reason is because water vapor by itself cannot increase temperatures; it can only amplify already-occurring warming. It is called a positive feedback mechanism. CO2, on the other hand, is a radiative forcing mechanism. CO2 CAN trigger increases in temperature.

To properly understand the role of water vapor in the climate system requires understanding the concept of humidity. The amount of water that the atmosphere can hold at any one time is called the humidity. The relative humidity (for a specific temperature at sea level) is a dimensionless number that measures the amount of water vapor found in a cubic meter of air compared to the  maximum amount of water vapor that cubic meter of air can hold. As temperature increases, relative humidity also increases. Another way of saying this is that the higher the temperature, the more water the atmosphere is capable of storing. No matter what the temperature, however, when the air is 100% saturated with water vapor, then it has reached its dew point and cannot hold another drop of water. Trying to exceed the dew point will result in precipitation, fog or mist.

While we can continue to increase the concentration of CO2 in the atmosphere, there is a natural limit to water vapor concentration. Trying to increase more water vapor in the atmosphere beyond its dew point is impossible and will only result in precipitation.

CO2 levels have increased from 0.028 percent of the atmosphere to about 0.04 percent since the Industrial Revolution. This has led to a temperature increase of about 0.7 degrees Celsius so far. The increased temperature is due to feedback effects with  water – causing more evaporation and hence higher concentrations of water vapor, which further increases temperature.

The IPCC estimates that about half of the additional warming could be due to feedback warming from water vapor but it would not have happened without the added CO2 pumped into the atmosphere. CO2 is like the guy robbing the bank while water vapor is just the getaway driver.


Emissions by plants to the atmosphere are influenced by climate change — higher temperatures can increase the rate of the biological processes that control the emissions. If natural emissions increase as the temperature rises, this in turn increases the amount of particles that are formed  

- Kent Salo of the Department of Chemistry at the University of Gothenburg

An aerosol is an airborne suspension of fine solid particles or liquid droplets in a gas. There are both natural and anthropogenic sources of aerosols. Examples or sources of aerosols are trees, emissions from incomplete combustion of fossil fuels and burning of trees and shrubs. They are short lived, often lasting only a few weeks but they play a major role in climate regulation in a process called global dimming. Generally speaking, these particles in the atmosphere have a cooling effect on the Earth. They affect cloud formation; a larger number of particles in the air leads to an increase in the number of cloud droplets. This affects the lifetime of the clouds and the amounts of precipitation, and consequently, the climate.

A major 2013 study cites black carbon, a type of anthropogenic aerosol created by incomplete combustion of fossil fuels, wood for cooking or agricultural burning as a major contributor to global warming. At the same time, because it is far easier to control than CO2 emissions, it provide an important short term method to easily lower global warming impacts while we are evolving more costly solutions and expensive decarbonization programs to transition from a fossil fuel to non-carbon economy.

Aerosols are problematic for global warming. While they are responsible for global warming, they are simultaneously responsible for global dimming, the blocking of solar radiation from reaching other parts of the earth system. Hence they play a role in both increasing and decreasing warming.

Net Energy Flux

The total amount of energy impinging upon the earth is estimated at 174 Petawatts. As mentioned above, an average albedo of 0.3 means that 30% is reflected back into space through the atmosphere, clouds and the earth’s surface and 70% is absorbed by oceans, land masses and clouds.

Figure 13: Earth’s longwave thermal radiation intensity, from clouds, atmosphere and ground (Source: NOAA)

 Figure 14: Earth’s energy budget (Source: NASA)

The energy that reaches the land and ocean surface of the planet are converted into longer wavelength infrared radiation and goes back out into space. Overall, 100% of the incoming solar energy is ultimately radiated back out as either reflection (30%) or transformed longer wavelength infrared radiation (70%). This net balance of net zero energy is reflected by the fact that the average temperature of the surface of the globe has remained 15 Deg. C for a long period of time.

Calculating the amount of incoming short wavelength radiation

Calculating the amount of outgoing long wavelength radiation

The Natural Greenhouse Effect

As mentioned above, a greenhouse gas is transparent to short wave radiation but opaque to long wave (infrared radiation). CO2, methane and water vapor are all considered greenhouse gases because they allow incident short wave solar energy to penetrate through to the surface of the planet. Yet when the longer wavelength infrared radiation tries to escape into outerspace, these gases are excited and begin to resonate at the infrared frequency.

Though they constitute 99% of atmospheric gases, Nitrogen and Oxygen are NOT greenhouse gases. They are transparent both to incoming sunlight as well as outgoing thermal infrared radiation. Therefore, they have no heat trapping ability.

Because greenhouse gas molecules radiate heat in all directions, some of it goes upwards to other layers of the atmosphere while other spreads downward and ultimately comes back into contact with the Earth’s surface, where it is absorbed. This downward component is called back radiation and it results in a substantial increase in energy and subsequently, surface temperature. This supplemental heating of the Earth’s surface by the atmosphere is the natural greenhouse effect.

Natural greenhouse effect responsible for surface temperature of 15 Deg. C

The solar energy measured at the top of the atmosphere (TOA) is 239 W/m2 and the average global temperature on the surface of the earth is 15 Deg. C.

Figure 15: Blackbody radiation curves

Through the famous blackbody radiation law, we can determine the temperature given the energy or vice versa.  239 W/m2 TOA corresponds to a temperature of 255 Deg K or -18 Deg C. Yet the average temperature on the surface of the earth  is 15 Deg C which, by blackbody radiation relation corresponds to 288 Deg K and 390 W/2.

This discrepancy is explained by the absorption of infrared energy by greenhouse gases found in the atmosphere.  Hence, greenhouse gases absorb and reflect energy back to the planet, raising the surface of the planet by about 30 Deg. C.

Disturbing the Natural Carbon Cycle

We are finally ready to put all the above pieces together to tell the story of how human civilizations increase of a trace atmospheric gas, CO2, can have such a powerful influence on the earth’s climate system. We first begin the story with the natural greenhouse effect.

Though only constituting 0.04% of atmosphere gases,  CO2 has a resonant frequency corresponding to 700 nm wavelength of the long wave infrared radiation emitted by the surface of the earth into space. The CO2 molecules are set into vibration, effectively raising the temperature of the atmosphere. Furthermore, when they collide with other atmospheric molecules such as Nitrogen, Oxygen and water molecules, they transfer their energy to them as well. In this way, the other molecules amplify the increased energy of CO2.  Hence, introducing more CO2 into the atmosphere increases the heat trapping ability of the planet, absorbing more energy that would otherwise be radiated out into space – like putting an extra blanket on when you are already overheated.

The videos below provide excellent explanations of these effects.

How CO2 triggers an increase in atmospheric temperature

CO2 and radiative forcing

Proving CO2 is anthropogenic

Finally, the effects of global warming are seen dramatically on this graph based on the Vostok ice core samples taken in 2002. It shows not only that CO2 track temperature changes quite tightly for the past 400,000 years, but also that the amount and speed of CO2 increase is unprecedented in the last 400,000 years. In fact, the last time it changed this quick was 65 million years previously when a meteor landed on the planet sending a huge dust plume into the atmosphere that ultimately caused the last mass extinction event.

Figure 16: Vostok ice core data

NOAA Time history of atmospheric carbon dioxide from 800,000 years before present until January, 2012. Recommend full screen/HD to read titles. (Source: NOAA)

Global Warming Potential (GWP)

Figure 17: Concentrations of GHG

There are a number of gases that can cause global warming. To find a common way to measure all these gases is through the Global Warming Potential (GWP) concept. This is the yardstick which compares any GHG warming potential to the reference of CO2 warming potential.

GWP is defined as the cumulative radiative forcing – both direct and indirect effects – integrated over a period of time from the emission of a unit mass of gas relative to CO2 as a reference gas (IPCC 1996). Carbon dioxide (CO2) was chosen by the IPCC as this reference gas and its GWP is set equal to one (1).

There are three key factors that determine the GWP value of a GHG:

  1. the gases absorption of infrared radiation,
  2. the gas absorbtion wavelength,
  3. the atmospheric lifetime of the gas

GWP only applies to gases that have a long atmospheric lifetime (i.e., in years).  Because only these gases last long enough in the atmosphere to mix evenly and spread throughout the atmosphere to form a relatively uniform concentration. GWP values are meant to be “global,” as the name implies. So if a gas is short-lived and does not have a global concentration because it is destroyed quickly and emitted in different amounts in different places, then it doesn’t have a GWP.

Figure 18: Radiation transmitted by the atmosphere (Source GHG Institute) 

The above figure shows a graph of the spectrum of wavelengths at which solar radiation interacts with the earth system. The sun emits short wave radiation shown in the figure in red. As it moves through the atmosphere, it is absorbed by a  number of gases as well as scattered. The same amount of energy that enters the planet must leave. This is called the earth’s energy budget. The energy leaves as long wavelength radiation (heat).  The gray graphs show the extent to which various major atmosphere components absorb energy or incoming solar radiation.

  1. Incoming shrt wave solar radiation passes through the atmosphere and reaches the earth (red)
  2. Outgoing long wave thermal energy radiates back from the earth out into space (blue)
  3. Atmospheric greenhouse gases act as an insulator. While they allow incoming short wave radiation to pass through, they prevent longer wavelength thermal radiation from escaping
  4.  (in gray) is an energy absorber which will absorb certain wavelengths of the short wave solar radiation in a given “window” of the spectrum
  5. The more that window is filled up, the less there is to absorb
  6. As concentrations of certain gases increase they can saturate that wavelength, leaving no more radiation for additional concentrations of gas in the atmosphere to absorb

The definition of GWP states that the absorption is performed over a period of time. The IPCC usually assumes this period is 100 years. However, given the immediate climate crisis, we don’t have more than a few decades to make fundamental changes. IPCC can measure over 20, 100 and 500 years. When we look at the table below and look up the GWP  for the 20 year interval, suddenly, the situation seems a lot worse. Methane, which is said to have 21x GWP (over a 100 year period) actually has a GWP of 72 over the first 20 year period.

The gases with major GWP are:

  • Carbon dioxide (CO2) – Fossil fuel use is the primary source of CO2. The way in which people use land is also an important source of CO2, especially when it involves deforestation. Land can also remove CO2 from the atmosphere through reforestation, improvement of soils, and other activities.
  • Methane (CH4) – Agricultural activities, waste management, and energy use all contribute to CH4emissions.
  • Nitrous oxide (N2O) – Agricultural activities, such as fertilizer use, are the primary source of N2O emissions.
  • Fluorinated gases (F-gases) – Industrial processes, refrigeration, and the use of a variety of consumer products contribute to emissions of F-gases, which include hydrofluorocarbons (HFCs), perfluorocarbons (PFCs), and sulfur hexafluoride (SF6).

 Table 1:
Global Warming Potential (GWP)
of various gases referenced to
IPCC 1996, 2001 and 2007
(Source GHG Institute)

Online Courses on Climate Change

Courses from Pacific Institute for Climate Solutions

Courses on introduction to climate change from PICS

Figure 19 : Sources of GHG (Source: WRI)

Climate Change Impacts

How Data From the Past is used to Construct Present Day Picture of Emissions


Elementa: Science of the Anthropocene is an open-source online science journal that reports on fundamental advancements in research organized initially into six knowledge domains, led by prominent researchers:

Atmospheric ScienceAtmospheric Science 
Detlev Helmig
University of Colorado Boulder

Donald R. Zak
University of Michigan

Elementa embraces the concept that basic knowledge can foster sustainable solutions for society. Elementa is published on an open-access, public-good basis—available freely and immediately to the world.