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
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 global warming.
The simplest explanation of the global warming problem is shown by the graphic below.
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(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 widget to the left measures the net heat energy measured in units of Hiroshima bombs that 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 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
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.
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.
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:
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:
- As early as 1965, a government committee warned President Johnson of the dangers of global warming
- 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
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
Steps in the Cycle
- In the atmosphere, carbon dioxide (CO2) and water react to form carbonic acid.
- 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
- These component ions are carried in surface waters like streams and rivers to the ocean, where they precipitate out as minerals like calcite (CaCO3)
- Through continued deposition and burial, this calcite sediment forms the rock called limestone
- This cycle continues as seafloor spreading pushes the seafloor under continental margins in the process of subduction
- 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
- This return to the atmosphere can occur violently through volcanic eruptions, or more gradually in seeps, vents, and CO2-rich hotsprings
- 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)
- 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 CO2 back 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 CO2 in 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:
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 howCO2 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:
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
, or Reflectivity
The planet’svaries 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 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:
- a mixture of gases
- water vapor
The mixture of gas consists of:
- 78% of nitrogen – N2
- 21% of oxygen – O2,
- 1% of argon,
- 1% of a mixture of trace gases including all the gases called greenhouse gases (GHG) such as CO2, Methane (CH4), Hydrofluorocarbons (HFC’s)
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 andsources 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 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 averageof 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)
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.
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
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:
- the gases absorption of infrared radiation,
- the gas absorbtion wavelength,
- 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.
- Incoming shrt wave solar radiation passes through the atmosphere and reaches the earth (red)
- Outgoing long wave thermal energy radiates back from the earth out into space (blue)
- 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
- (in gray) is an energy absorber which will absorb certain wavelengths of the short wave solar radiation in a given “window” of the spectrum
- The more that window is filled up, the less there is to absorb
- 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).
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
- Lesson 1: CO2 and Greenhouse Effect
- Lesson 2: Mother Nature’s Influence
- Lesson 3: Observable Changes
- Lesson 4: Climate Modeling
Figure 19 : Sources of GHG (Source: WRI)
Climate Change Impacts
How Data From the Past is used to Construct Present Day Picture of Emissions
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