What is Climate Sensitivity?
Climate sensitivity describes how sensitive the global climate is to a radiative forcing. For example, we know that if the amount of carbon dioxide (CO2) in the Earth's atmosphere doubles from the pre-industrial level of 280 parts per million by volume (ppmv) to 560 ppmv, this will cause an energy imbalance (trapping more radiation, causing more incoming than outgoing energy on Earth) enough to directly heat the planet about 1.2°C. However, this doesn't account for feedbacks, for example ice melting and making the planet less reflective, and the warmer atmosphere holding more water vapor (another greenhouse gas).
Climate sensitivity tells us the amount the planet will warm when accounting for all of these feedbacks. The relevant formula is:
dT = λ*dF
Where 'dT' is the change in the Earth's average surface temperature, 'λ' is the climate sensitivity, usually with units in Kelvin or degrees Celsius per Watts per square meter (°C/[W m-2]), and 'dF' is the radiative forcing.
Climate sensitivity is not specific to CO2
A common misconception is that the climate sensitivity and temperature change in response to increasing CO2 is different than the sensitivity to other radiative forcings, like a change in solar irradiance. However, this is not the case. Climate sensitivity is a set value (approximately 0.8°C/[W m-2]), and the surface temperature change is proportional to the sensitivity and radiative forcing (in W m-2), regardless of the source of the radiative forcing. In other words, if you're arguing for a low climate sensitivity to CO2, you're also arguing for a low climate sensitivity to solar irradiance, orbital changes, volcanic emissions, and everything else. Thus when arguing for low climate sensitivity, it becomes difficult to explain past climate changes. For example, between glacial and interglacial periods, the planet's temperature changes on the order of 10°C. If the climate sensitivity is low, for example due to increasing low-lying cloud cover reflecting more sunlight as a response to global warming, then how can these large past climate changes be explained?
What are the known feedbacks?
There are many known feedbacks. Some of them are specific to increasing atmospheric CO2, and some are simply a response to a warming average global temperature regardless of the cause. Since we're concerned about the climate sensitivity to increasing atmospheric CO2, here is a list of some of the main feedbacks in this scenario.
- Melting Ice. Ice is a highly reflective material, whereas oceans and soil tend to be dark and absorbent. The more the planet warms, the more ice melts, the less reflective the Earth becomes as a whole (also known as decreasing albedo), and the more solar energy it absorbs, causing further warming.
- Carbon release from melting permafrost. There are large quantities of carbon trapped beneath the Earth's permafrost, which can be released as CO2 or methane (a greenhouse gas 23 times more potent than CO2). Recent estimates put these deposits at over 1.5 trillion tons of frozen carbon, about twice as much carbon as contained in the atmosphere. The melting of permafrost and subsequent carbon release has already begun, and obviously most of this carbon will accumulate in the atmosphere, increasing the greenhouse effect and causing further warming.
- Carbon release from warming peat bogs. Higher global temperatures could cause water tables to drop substantially, causing more peat to dry and decompose. There is a total of 200 to 450 billion metric tons of carbon sequestered in peat bogs worldwide, and as with melting permafrost, some is released as CO2 and some as methane.
- Saturating oceans. The oceans are the largest carbon sink (storage medium) on the planet, but there's a limit to how much carbon they can store. In fact a recent study found that "the rate of CO2 accumulation in the deepest basin of the East/Japan Sea has considerably decreased over the transition period between 1992-1999 and 1999-2007." As the oceans saturate, they'll be able to absorb less carbon, and more CO2 will accumulate in the atmosphere, causing further global warming.
- Impacts to forests. As the planet warms and there are fewer long cold snaps during the winter, more pine beetles survive to wreak havoc on forests during the summer. One recent study of a British Columbia forest estimated that the cumulative impact of the beetle outbreak in that region alone during 2000–2020 will be 270 megatonnes (Mt) of carbon. The study concluded that a recent pine beetle outbreak "converted the forest from a small net carbon sink to a large net carbon source both during and immediately after the outbreak. In the worst year, the impacts resulting from the beetle outbreak in British Columbia were equivalent to 75% of the average annual direct forest fire emissions from all of Canada during 1959–1999."
- Increasing atmospheric water vapor. Much ado is made about water vapor, as it's responsible for approximately 3-4 times more of the greenhouse effect on Earth than CO2. However, the amount of water vapor in the atmosphere is strictly regulated by the temperature of the atmosphere. Thus as the planet and its atmosphere warms, the amount of water vapor increases, causing further warming yet.
There are many more positive feedbacks (causing more warming), but these are the primary ones. In contrast, there are few known potentially significant negative feedbacks (causing cooling in response to warming).
- Low cloud cover. Low-altitude clouds increase the Earth's overall reflectivity (albedo) and thus cause a cooling effect. The question is, how will the amount of low cloud cover respond to a warming world? There is much debate and uncertainty about this subject. One recent study of cloud cover in the northeastern Pacific Ocean found that the only "particularly realistic" global climate model (GCM) in modeling the changes in cloud cover they observed was the GCM with the highest climate sensitivity (4.4°C for a doubling of CO2), "providing modeling evidence for a positive low-level cloud feedback." However, this was a regional study, and may not accurately represent the low cloud cover response on a global scale.
What is the possible range of climate sensitivity?
The IPCC Fourth Assessment Report put the possible range of climate sensitivity at likely to be in the range 2 to 4.5°C with a best estimate of about 3°C, and is very unlikely to be less than 1.5°C. Values substantially higher than 4.5°C cannot be excluded, but agreement of models with observations is not as good for those values.
Individual studies have put climate sensitivity to a doubling of CO2 from anywhere between 0.5°C and 10°C, but the higher and lower values are very unlikely. In fact over time as climate science has developed and advanced, estimates have converged around 3°C. A summary of recent climate sensitivity studies can be found here. Climate scientist Stefan Rahmstorf stated in a 2008 study "many vastly improved models have been developed by a number of climate research centers around the world. Current state-of-the-art climate models span a range of 2.6–4.1°C, most clustering around 3°C." Several studies have put the lower bound of climate sensitivity at about 1.5°C, but several others have found that a sensitivity higher than 4.5°C can't be ruled out.
A 2008 study by James Hansen found that climate sensitivity to "fast feedback processes" is 3°C, but when accounting for longer-term feedbacks (mainly decreasing albedo from melting ice), if atmospheric CO2 remains at the doubled level, the sensitivity increases to 6°C based on paleoclimate (historical climate change) data.
What are the limits on the value of climate sensitivity?
The main limit on the sensitivity value is that it has to be consistent with paleoclimate data. A sensitivity which is too low will be inconsistent with past climate changes - basically if there is some large negative feedback which makes the sensitivity too low, it would have prevented the planet from transitioning from ice ages to interglacial periods, for example. Similarly a high climate sensitivity would have caused more and larger past climate changes.
One recent study examining the Palaeocene–Eocene Thermal Maximum (about 55 Myr ago), during which the planet warmed 5-9°C, found that "At accepted values for the climate sensitivity to a doubling of the atmospheric CO2 concentration, this rise in CO2 can explain only between 1 and 3.5 °C of the warming inferred from proxy records." This suggests that climate sensitivity may be higher than we currently believe, but it likely isn't lower.
Recent responses to large volcanic eruptions
Climate scientists have also attempted to estimate climate sensitivity based on the response to recent large volcanic eruptions, such as Mount Pinatubo in 1991. Wigley et al. (2005) found:
Similarly, Forster et al. (2006) concluded as follows.
Other Empirical Observations
Gregory et al. (2002) used observed interior-ocean temperature changes, surface temperature changes measured since 1860, and estimates of anthropogenic and natural radiative forcing of the climate system to estimate its climate sensitivity. They found:
Examining Past Temperature Projections
In 1988, NASA climate scientist James Hansen produced a groundbreaking study in which he used a global climate model to project how much the climate would warm in the future based on 3 different human CO2 emissions scenarios (A, B, and C). A couple of decades later we can now go back and examine his assumptions and results, and see what this tells us about the climate sensitivity.
As he notes in Section 2 on page 2 of the study, Hansen's model assumed a rather high climate sensitivity of 4.2°C for a doubling of CO2. His Scenario B has been the closest to reality, with the total radiative forcing in reality being about 10% higher than in this emissions scenario. The warming trend predicted in this scenario from 1988 to 2010 was about 0.26°C per decade whereas the measured temperature increase over that period was approximately 0.18°C per decade, or about 40% lower than Scenario B. Therefore, what James Hansen's projections and assumptions and the measured real-world changes tell us is that climate sensitivity is about 40% below 4.2°C, or once again, right around 3°C for a doubling of atmospheric CO2.
Probabilistic Estimate Analysis
Annan et al. (2009) investigated various probabilistic estimates of climate sensitivity, many of which suggested a "worryingly high probability" (greater than 5%) that the sensitivity is greater than 6°C for a doubling of CO2. Using a Bayesian statistical approach, this study concluded that
Annan concludes that the climate sensitivity to a doubling of atmospheric CO2 is probably close to 3°C, it may be higher, but it's probably not much lower.
What does all this mean?
According to a recent MIT study, we're currently on pace to reach this doubled atmospheric CO2 level by around 2050.
Projected decadal mean concentrations of CO2. Red solid lines are median, 5% and 95%
percentiles for present study, dashed blue line the same from the MIT 2003 projection.
So unless we change course, we're looking at a rapid warming over the 21st century. Most climate scientists agree that a 2°C warming is the 'danger limit'. As the climate scientists at RealClimate put it, "Global warming of 2°C would leave the Earth warmer than it has been in millions of years, a disruption of climate conditions that have been stable for longer than the history of human agriculture. Given the drought that already afflicts Australia, the crumbling of the sea ice in the Arctic, and the increasing storm damage after only 0.8°C of warming so far, calling 2°C a danger limit seems to us pretty cavalier."