"How carbon dioxide controls earth’s temperature"
NASA’s Lacis: “There is no viable alternative to counteract global warming except through direct human effort to reduce the atmospheric CO2 level.”
A study by GISS climate scientists recently published in the journal Science shows that atmospheric CO2 operates as a thermostat to control the temperature of Earth….
CO2 is the key atmospheric gas that exerts principal control (80% of the non-condensing GHG forcing) over the strength of the terrestrial greenhouse effect. Water vapor and clouds are fast-acting feedback effects, and as such, they are controlled by the radiative forcing supplied by the non-condensing GHGs….
There is no viable alternative to counteract global warming except through direct human effort to reduce the atmospheric CO2 level.
NASA’s Goddard Institute for Space Studies has posted three articles on their website explaining two important new studies, “Atmospheric CO2: Principal control knob governing Earth’s temperature” (subs. req’d) in Science by Andrew Lacis et al. and “The attribution of the present-day total greenhouse effect” (subs. req’d) in JGR by Gavin Schmidt et al. Together they make a terrific tutorial on the critical role human-caused CO2 plays in climate change.
Schmidt is best known as a key contributor to the must-read blog, Real Climate. Lacis may be best known as the NASA climatologist whose 2005 critique of the IPCC Fourth Assessment draft — “There is no scientific merit to be found in the Executive Summary” — was embraced by the anti-science disinformers until it was revealed he thought the IPCC consensus was in fact some watered down, least-common denominator piece of wishy-washiness that understates our scientific understanding, which it is (see “Disputing the ‘consensus’ on global warming“).
It may be obvious to CP readers and all those who follow the science, but the core conclusion of the Science article bears repeating again and again by all of us who communicate on global warming:
Ample physical evidence shows that carbon dioxide (CO2) is the single most important climate-relevant greenhouse gas in Earth’s atmosphere. This is because CO2, like ozone, N2O, CH4, and chlorofluorocarbons, does not condense and precipitate from the atmosphere at current climate temperatures, whereas water vapor can and does. Noncondensing greenhouse gases, which account for 25% of the total terrestrial greenhouse effect, thus serve to provide the stable temperature structure that sustains the current levels of atmospheric water vapor and clouds via feedback processes that account for the remaining 75% of the greenhouse effect. Without the radiative forcing supplied by CO2 and the other noncondensing greenhouse gases, the terrestrial greenhouse would collapse, plunging the global climate into an icebound Earth state.
Indeed, absent greenhouse gases, the planet would be about 60°F colder.
Here is NASA’s Research News piece, “How Carbon Dioxide Controls Earth’s Temperature” summarizing the two studies:
Water vapor and clouds are the major contributors to Earth’s greenhouse effect, but a new atmosphere-ocean climate modeling study shows that the planet’s temperature ultimately depends on the atmospheric level of carbon dioxide.
The study, conducted by Andrew Lacis and colleagues at NASA’s Goddard Institute for Space Studies (GISS) in New York, examined the nature of Earth’s greenhouse effect and clarified the role that greenhouse gases and clouds play in absorbing outgoing infrared radiation. Notably, the team identified non-condensing greenhouse gases “” such as carbon dioxide, methane, nitrous oxide, ozone, and chlorofluorocarbons “” as providing the core support for the terrestrial greenhouse effect.
Without non-condensing greenhouse gases, water vapor and clouds would be unable to provide the feedback mechanisms that amplify the greenhouse effect. The study’s results will be published Friday, Oct. 15, in Science.
A companion study led by GISS co-author Gavin Schmidt that has been accepted for publication in the Journal of Geophysical Research shows that carbon dioxide accounts for about 20 percent of the greenhouse effect, water vapor and clouds together account for 75 percent, and minor gases and aerosols make up the remaining five percent. However, it is the 25 percent non-condensing greenhouse gas component, which includes carbon dioxide, that is the key factor in sustaining Earth’s greenhouse effect. By this accounting, carbon dioxide is responsible for 80 percent of the radiative forcing that sustains the Earth’s greenhouse effect.
The climate forcing experiment described in Science was simple in design and concept “” all of the non-condensing greenhouse gases and aerosols were zeroed out, and the global climate model was run forward in time to see what would happen to the greenhouse effect.
Without the sustaining support by the non-condensing greenhouse gases, Earth’s greenhouse effect collapsed as water vapor quickly precipitated from the atmosphere, plunging the model Earth into an icebound state “” a clear demonstration that water vapor, although contributing 50 percent of the total greenhouse warming, acts as a feedback process, and as such, cannot by itself uphold the Earth’s greenhouse effect.
“Our climate modeling simulation should be viewed as an experiment in atmospheric physics, illustrating a cause and effect problem which allowed us to gain a better understanding of the working mechanics of Earth’s greenhouse effect, and enabled us to demonstrate the direct relationship that exists between rising atmospheric carbon dioxide and rising global temperature,” Lacis said.
The study ties in to the geologic record in which carbon dioxide levels have oscillated between approximately 180 parts per million during ice ages, and about 280 parts per million during warmer interglacial periods. To provide perspective to the nearly 1°C (1.8°F) increase in global temperature over the past century, it is estimated that the global mean temperature difference between the extremes of the ice age and interglacial periods is only about 5°C (9°F).
“When carbon dioxide increases, more water vapor returns to the atmosphere. This is what helped to melt the glaciers that once covered New York City,” said co-author David Rind, of NASA’s Goddard Institute for Space Studies. “Today we are in uncharted territory as carbon dioxide approaches 390 parts per million in what has been referred to as the ‘superinterglacial.’”
“The bottom line is that atmospheric carbon dioxide acts as a thermostat in regulating the temperature of Earth,” Lacis said. “The Intergovernmental Panel on Climate Change has fully documented the fact that industrial activity is responsible for the rapidly increasing levels of atmospheric carbon dioxide and other greenhouse gases. It is not surprising then that global warming can be linked directly to the observed increase in atmospheric carbon dioxide and to human industrial activity in general.”
Here is Lacis’s explanatory Science Brief, “The Thermostat that Controls Earth’s Temperature“:
A study by GISS climate scientists recently published in the journal Science shows that atmospheric CO2 operates as a thermostat to control the temperature of Earth.
There is a close analogy to be drawn between the way an ordinary thermostat maintains the temperature of a house, and the way that atmospheric carbon dioxide (and the other minor non-condensing greenhouse gases) control the global temperature of Earth. The ordinary thermostat produces no heat of its own. Its role is to switch the furnace on and off, depending on whether the house temperature is lower or higher than the thermostat setting. If we were to carefully monitor the temperature of the house, we would see that the temperature does not stay constant at the set value, but rather exhibits a “natural variability” as the house temperature slips below the set value and then overshoots the mark with a time constant of minutes to tens of minutes, because of the thermal inertia of the house and because heating by the furnace (when it is on) is more powerful than the steady heat loss to the outdoors. If the thermostat is suddenly turned to a very high setting, the temperature will begin to rise at a rate dictated by the inertia of the house and strength of the furnace. Turning the thermostat back to normal will stop the heating.
Atmospheric carbon dioxide performs a role similar to that of the house thermostat in setting the equilibrium temperature of the Earth. It differs from the house thermostat in that carbon dioxide itself is a potent greenhouse gas (GHG) warming the ground surface by means of the greenhouse effect. It is this sustained warming that enables water vapor and clouds to maintain their atmospheric distributions as the so-called feedback effects that amplify the initial warming provided by the non-condensing GHGs, and in the process, account for the bulk of the total terrestrial greenhouse effect. Since the radiative effects associated with the buildup of water vapor to near-saturation levels and the subsequent condensation into clouds are far stronger than the equilibrium level of radiative forcing by the non-condensing GHGs, this results in large local fluctuations in temperature about the global equilibrium value. Together with the similar non-linear responses involving the ocean heat capacity, the net effect is the “natural variability” that the climate system exhibits regionally, and on inter-annual and decadal timescales, whether the global equilibrium temperature of the Earth is being kept fixed, or is being forced to re-adjust in response to changes in the level of atmospheric GHGs.
Radiative modeling analyses of the terrestrial greenhouse structure described in a parallel study in the Journal of Geophysical Research (Schmidt et al., 2010) found that water vapor accounts for about 50% of the Earth’s greenhouse effect, with clouds contributing 25%, carbon dioxide 20%, and the minor greenhouse gases (GHGs) and aerosols accounting for the remaining 5%, as shown in Fig. 1.
Figure 1. Attribution of individual atmospheric component contributions to the terrestrial greenhouse effect, separated into feedback and forcing categories. Dotted and dashed lines depict the fractional response for single-addition and single-subtraction of individual gases to either an empty or full-component reference atmosphere, respectively. Solid black lines are the scaled averages of the dashed and dotted line fractional response results. The sum of the fractional responses must add up to the total greenhouse effect. The reference model atmosphere is for 1980 conditions.
Thus, while the non-condensing greenhouse gases account for only 25% of the total greenhouse effect, it is these non-condensing GHGs that actually control the strength of the terrestrial greenhouse effect since the water vapor and cloud feedback contributions are not self-sustaining and as such, only provide amplification. Because carbon dioxide accounts for 80% of the non-condensing GHG forcing in the current climate atmosphere, atmospheric carbon dioxide therefore qualifies as the principal control knob that governs the temperature of Earth.The numerical climate experiment described in Fig. 2. demonstrates the fundamental radiative forcing role of the non-condensing GHGs, and the feedback (only) role of water vapor and clouds. This climate modeling experiment was performed using the GISS ModelE general circulation coupled atmosphere-ocean climate model by zeroing out all of the non-condensing greenhouse gases. Doing this removed the radiative forcing that sustains the temperature support for water vapor and cloud feedbacks, causing rapid condensation and precipitation of water vapor from the atmosphere, collapsing the terrestrial greenhouse effect, and plunging the Earth into an icebound state.
Figure 2. Time evolution of global surface temperature, top-of-atmosphere (TOA) net flux, column water vapor, planetary albedo, sea ice cover, and cloud cover, after zeroing out all the non-condensing greenhouse gases. The model used in the experiment is the GISS 2°—2.5° AR5 version of ModelE with the climatological (Q-flux) ocean energy transport and the 250 m mixed layer depth. The model initial conditions are for a pre-industrial atmosphere. Surface temperature and TOA net flux utilize the left-hand scale.
The scope of the climate impact becomes apparent in just 10 years. During the first year alone, global mean surface temperature falls by 4.6 °C. After 50 years, the global temperature stands at -21 °C, a decrease by 34.8 °C. Atmospheric water vapor is at ~10% of the control climate value (22.6 to 2.2 mm). Global cloud cover increases from its 58% control value to more than 75%; the global sea ice fraction goes from 4.6% to 46.7%, causing the planetary albedo of Earth to increase from ~29% to 41.8%. This has the effect to reduce the absorbed solar energy to further exacerbate the global cooling. After 50 years, one third of the ocean surface still remains ice-free, even though the global surface temperature is colder than -21 °C. At tropical latitudes, incident solar radiation is enough to keep the ocean from freezing. While this thermal oasis within an otherwise icebound Earth appears to be stable, at least on the short timescale illustrated, further calculations with an interactive ocean are needed to verify the potential for long-term stability. The surface temperatures in Fig. 3 are only marginally warmer than 1 °C within the remaining low latitude heat island.
Figure 3. Zonally averaged annual-mean surface temperature change following the zeroing out of non-condensing greenhouse gases.
From the foregoing, it is clear that CO2 is the key atmospheric gas that exerts principal control (80% of the non-condensing GHG forcing) over the strength of the terrestrial greenhouse effect. Water vapor and clouds are fast-acting feedback effects, and as such, they are controlled by the radiative forcing supplied by the non-condensing GHGs.
The bottom line is that atmospheric carbon dioxide acts as a thermostat in regulating the temperature of Earth. The rapid increase in atmospheric carbon dioxide due to human industrial activity is therefore setting the course for continued global warming. Because of the large heat capacity of the climate system, the global surface temperature does not respond instantaneously to the sharp upturn of the carbon dioxide thermostat, which at this moment stands at 386.80 ppm compared to the normal interglacial maximum level of 280 ppm. Since humans are responsible for changing the level of atmospheric carbon dioxide, they then also have control over the global temperature of the Earth. Humans are at a difficult crossroad. Carbon dioxide is the lifeblood of civilization as we know it. It is also the direct cause fueling an impending climate disaster. There is no viable alternative to counteract global warming except through direct human effort to reduce the atmospheric CO2 level.
The basic physics for the present study is rooted in the high precision measurements documenting the rise of atmospheric carbon dioxide and other greenhouse gases as fully described in the IPCC AR4 report, and in the comprehensive HITRAN database (Rothman et al. 2009) of atmospheric absorption data. The radiative transfer calculations involve well-understood physics that is applied to the global energy balance of the Earth, which is maintained by radiative processes only, since the global net energy transports must equal zero. This demonstrates the nature of the terrestrial greenhouse effect as being sustained by the non-condensing GHGs, with magnification of the greenhouse effect by water vapor and cloud feedbacks, and leaves no doubt that increasing GHGs cause global warming.
For an excellent talk on the role of CO2, see In must-see AGU video, Richard Alley explains “The Biggest Control Knob: Carbon Dioxide in Earth’s Climate History.”
Here is Schmidt’s explanatory Science Brief, “Taking the Measure of the Greenhouse Effect“:
Most of us have heard that the greenhouse effect keeps the planet much warmer than it would be otherwise, and similarly we may have heard that increasing amounts of greenhouse gases are enhancing the natural greenhouse effect. But few of us appreciate what exactly it is in the atmosphere that makes the effect work and why small changes in trace gases such as carbon dioxide (CO2) might make a difference.
It has been understood since the 19th century that some gases absorb infrared radiation (IR) that is emitted by the planet, slowing the rate at which the planet can cool and warming the surface. These so-called greenhouse gases include carbon dioxide and water vapor, as well as ozone and methane among others. Note, however, that the bulk of the atmosphere is made up of nitrogen and oxygen molecules which don’t absorb IR at all. Less well appreciated is that clouds (made of ice particles and/or liquid water droplets) also absorb infrared radiation and contribute to the greenhouse effect, too. Clouds, of course, also interfere with incoming sunlight, reflecting it back out to space, making their net effect one of cooling, but their contribution to the greenhouse effect is important.
The size of the greenhouse effect is often estimated as being the difference between the actual global surface temperature and the temperature the planet would be without any atmospheric absorption, but with exactly the same planetary albedo, around 33°C. This is more of a “thought experiment” than an observable state, but it is a useful baseline. Another way of quantifying the effect is to look at the difference between the infrared radiation emitted at the surface of the Earth, and the amount that is emitted to space at the top of the atmosphere. In the absence of the greenhouse effect, this would be zero (in other words, no difference). In actuality the surface emits about 150 Watts per square meter (W/m2) more than goes out to space.
So of all the greenhouse substances in the atmosphere, which of them absorbs what? This is a more complicated issue than it might first appear because of the nature of the absorption and the complex distribution of absorbers both horizontally and vertically. Different substances absorb different frequencies of IR, and the different parts of the planet differ wildly in how much IR is being emitted (based as it is on surface temperature) and how much cloud and water vapor there is at that location (carbon dioxide is very well mixed). Indeed, some wavelengths of IR can be absorbed by both water vapor or clouds, or water vapor and CO2. This “spectral overlap” means that if you remove a substance, the change in how much IR is absorbed will be less than if you only had that substance in the air. Alternately, the impact of all the substances together is less than what you would get if you added up their individual components. This needs to be taken into account in any attribution of the greenhouse effect.
Outgoing spectral radiance at the top of Earth’s atmosphere showing the absorption at specific frequencies and the principle absorber. For comparison, the red curve shows the flux from a classic “blackbody” at 294°K (‰ˆ31°C ‰ˆ 69.5°F). (View larger image)
We use the GISS model of radiative transfer through the global atmosphere to try and break down the attribution using realistic distributions of local temperature, water vapor and clouds. By removing each of the absorbers in turn and calculating the absorption for many different combinations, we can calculate all the overlaps and allocate the absorption fairly. We find that water vapor is the dominant substance “” responsible for about 50% of the absorption, with clouds responsible for about 25% “” and CO2 responsible for 20% of the effect. The remainder is made up with the other minor greenhouse gases, ozone and methane for instance, and a small amount from particles in the air (dust and other “aerosols”).
Given that CO2 has such a major role in the natural greenhouse effect, it makes intuitive sense that changes in its concentration because of human activities might significantly enhance the greenhouse effect. However, calculating the impact of a change in CO2 is very different from calculating the current role with respect to water vapor and clouds. This is because both of these other substances depend on temperatures and atmospheric circulation in ways that CO2 does not. For instance, as temperature rises, the maximum sustainable water vapor concentration increases by about 7% per degree Celsius. Clouds too depend on temperature, pressure, convection and water vapor amounts. So a change in CO2 that affects the greenhouse effect will also change the water vapor and the clouds. Thus, the total greenhouse effect after a change in CO2 needs to account for the consequent changes in the other components as well. If, for instance, CO2 concentrations are doubled, then the absorption would increase by 4 W/m2, but once the water vapor and clouds react, the absorption increases by almost 20 W/m2 “” demonstrating that (in the GISS climate model, at least) the “feedbacks” are amplifying the effects of the initial radiative forcing from CO2 alone. Past climate data suggests that this is what happens in the real world as well.
What happens when the trace greenhouse gases are removed? Because of the non-linear impacts of CO2 on absorption, the impact of removing the CO2 is approximately seven times as large as doubling it. If such an event were possible, it would lead to dramatic cooling, both directly and indirectly, as the water vapor and clouds would react. In model experiments where all the trace greenhouse gases are removed the planet cools to a near-Snowball Earth, some 35°C cooler than today, as water vapor levels decrease to 10% of current values, and planetary reflectivity increases (because of snow and clouds) to further cool the planet.
Despite being a trace gas, there is nothing trivial about the importance of CO2 for today, nor its role in shaping climate change in the future.
A satellite map of the outgoing longwave radiation emitted by Earth in September 2008 demonstrates not only geographical variations but also those caused by cloud presence. More heat escapes from areas just north and south of the equator, where the surface is warmer and there are fewer clouds. (Image: NASA/Earth Observatory/Robert Simmon from CERES data.)
For more on attribution, see “10 indicators of a human fingerprint on climate change.”
- U.S. National Academy of Sciences labels as “settled facts” that “the Earth system is warming and that much of this warming is very likely due to human activities”
- 1700 UK scientists come forward to reaffirm climate science
- 255 National Academy of Sciences members, including 11 Nobel laureates, defend climate science integrity: “There is compelling, comprehensive, and consistent objective evidence that humans are changing the climate in ways that threaten our societies and the ecosystems on which we depend.”
- American Meteorological Society reaffirms “that the atmosphere, ocean, and land surface are warming; that humans have significantly contributed to this change; and that further climate change will continue to have important impacts on human societies”¦.”
- The American Association for the Advancement of Science reaffirms “The scientific evidence is clear: global climate change caused by human activities is occurring now, and it is a growing threat to society.”