What Is Climate Sensitivity? Definition and Examples

Climate sensitivity is the term used by scientists to express the relationship between human-caused carbon dioxide (CO2) emissions and other greenhouse gases, and how that will affect temperature changes on Earth. This area specifically focuses on how much the Earth's temperature will increase with a doubling of greenhouse gases after various planetary forces have reacted to those increases and settled into a "new normal." Climate sensitivity is the term used by the Intergovernmental Panel of Climate Change (IPCC), the UN agency tasked with providing "regular scientific assessments on climate change, its implications and potential future risks." It puts this planet-wide change into a simple phrase so researchers can use it — and all its implications, feedbacks, and variabilities — as shorthand for the larger set of ideas.

Since pre-industrial times, CO2 has increased from levels of 280 parts per million (ppm) to 409.8 ppm in 2019. Researchers know with certainty that human beings weren't responsible for the amount of carbon or other greenhouse gases in the atmosphere before we started burning them at the start of the industry, which is considered the historic benchmark. Since the 1950s, CO2 measurements have come from the Moana Loa Volcanic Observatory; prior to that, they are found by taking measurements of trapped gas in ice cores. Projections put emissions at 560 ppm by around 2060 — that's double pre-industrial levels.

Climate sensitivity can be expressed as an equation that takes into account the average change in the surface temperatures of the Earth, accounting for the difference between ingoing and outgoing energy. Using that equation, climate sensitivity can be calculated as 3 degrees C — with the range of uncertainty being 2 to 4.5 degrees, meaning that's what the most robust models indicate will be the temperature change if CO2 doubles.

What Is the Climate Sensitivity Parameter?

The climate sensitivity parameter is an equation used to show where the specific numbers and predictions for the term come from. Because of the complexities of the global climate system, scientists can't just predict future warming and its effects based on what has happened in the past. Those complexities include feedback loops that will accelerate warming once certain benchmarks are passed; land use changes; and the influence of air pollution/particulate matter may have on shorter-term changes in climate.

If scientists want to figure out how much warming can be attributed to CO2 levels, they need an equation that takes into account as many variables as possible, while at the same time, keeping the calculations relatively simple. There are a few different equations that tackle this question.

This first equation is a simple one that doesn't include any feedbacks.

In Dave Burton's equation, S equals climate sensitivity, the number we are solving for. A is the attribution to human-caused CO2, which is 50% so .5 in the equation. T1 is the initial global average temperature for the time period you're choosing, and T2 is the final global average temperature. C1 is the initial CO2 value and C2 is the final value.

So, for example, let's look at the time period of 1960 (CO2 at 317 ppm) through 2014 (CO2 at 399 ppm). During that time, temperatures rose by .5°C at the low end, or .75°C at the higher end, so take the midpoint of those two numbers and use .625 degrees.

So T1 is 0 and T2 is 0.625

C1 is 317 (in 1960), C2 is 399 (in 2015) and A is 50%, then:

S = 0.5 × (0.625-0) / ((log(399)-log(317))/log(2))
     We can use Google as a calculator to find:
S = 0.94 °C / doubling

That means each doubling of CO2 will result in .94°C warming. That nearly 1 degree of warming is what most scientists agree would happen if the Earth's systems were static and there were no feedbacks.

Accounting for those feedbacks is important for understanding climate sensitivity. How impactful those feedbacks are — and how to weight them for inclusion in a climate sensitivity equation — is what climate scientists disagree on.

For example, here is another climate sensitivity equation that accounts for radiative forcing.

In this equation, climate sensitivity is the change in average temperatures multiplied by the radiative forcing resulting from a doubling of CO2 divided by the change in radiative forcing.

Different Methods to Estimate Climate Sensitivity

The formulas above are not the only climate sensitivity formulas. A well-known paper by Nicholas Lewis and Judith Curry includes estimates of radiative forcing and planetary heat uptake in their calculations. Other papers by scientists have weighted different aspects of the equation a little differently, with varying results.

Although all the formulas are asking and answering the same question, they each take into account different variables. There are dozens of other, similar equations that climate scientists use, and the numbers input for variables are regularly updated as more information is known.

What's important is that, even with all these different variables, climate scientists' answers to the various equations generally fall into the range mentioned as the IPCC number: With a doubling of CO2 in the atmosphere, a change of 2.5 to 4 degrees with an average of about 3 degrees is expected.

Radiative Forcing

Radiative forcing is the scientific way to describe the imbalance between the radiation that goes out and comes into the earth at the highest levels of the atmosphere.

When radiative forcing changes, it affects the temperature of Earth. This, in turn, influences the climate sensitivity equation — which is why it's such an important factor in understanding climate sensitivity.

Radiative forcing is affected by a few factors. One is the natural variability in solar radiation, such as fluctuations that depend on where the Earth is in its orbit around the sun, as well as solar flares and other changes in the sun's output.

The greenhouse effect, which creates conditions that increase how much radiation comes into the atmosphere, and aerosols, which can cause changes in cloud cover (which then can increase or decrease radiation) also affect radiative forcing.

Finally, land-use changes, like melting ice and snow in glaciers; permafrost; and deforestation can also influence how much radiative forcing happens.

Climate Feedbacks

Climate feedbacks are a really important part of the climate sensitivity puzzle. Feedback simply means that when one thing changes, it impacts another, which then alters the first thing in some way. These are internal parts of the process (unlike radiative forcing, which mostly comes from outside the system).

Some of these feedbacks can be challenging for scientists to pull out or isolate, because they are so closely tied to how the whole climate system works, while other feedbacks are isolated enough that it's fairly simple to account for how their changes impact the overall climate.

A runaway feedback loop has forces that are so strong that the effects of the first thing changing sets off a fast and intense feedback that happens much more quickly than other types of feedback loops.

There are a number of processes that can either exacerbate warming once it has begun (here called positive feedbacks, since they are accelerating the process), or do the opposite, cooling the climate (negative feedbacks, since they are slowing it down). Below are examples of positive feedback.

Permafrost Melting

Permafrost is the layer of soil or rock in mostly Arctic locations that stays frozen year-round. Some permafrost is at surface level, while other permafrost is below a layer that freezes and thaws seasonally.

When permafrost thaws due to increasing temperatures caused by climate change — this is happening in polar regions, which are warming twice as fast other areas of the Earth) — permafrost can release both CO2 and methane. This can occur when frozen peat bogs melt, like those in Western Siberia, which formed 11,000 years ago. Methane is a greenhouse gas that causes warming at levels 25 times higher than CO2, so if the methane contained in the peat bogs is released, it will contribute to further warming, which will melt more permafrost, and the cycle goes on.

A 2019 report from the National Oceanic and Atmospheric Administration reports that northern permafrost regions contain almost twice as much carbon as is currently in the atmosphere, and that this melting has already begun, creating what could be a runaway feedback loop.

Decomposition Imbalances

In mid-latitude regions, global warming trends will also increase methane released from freshwater ecosystems and wetlands. This is due to the warmer temperatures increasing the natural methane production of the microbial communities that live there. The tropics are predicted to get wetter as climate change progresses, and the soils there will decompose faster, limiting their ability to store carbon. Carbon sinks, like soils, are important to keep CO2 locked up, protected from being released into the atmosphere.

Lower water tables driven by warming mean that peat bogs will dry out. Some will burn, releasing methane, while others will dry out, which releases CO2. The dryer peat is also less able to store carbon in the future.

Drier Rainforests

Rainforests are very susceptible to climate changes as their natural balance is easily thrown off. So while some rainforest ecosystems will collapse under significant warming, it isn't just the loss of the forests that's of concern — the trees and other vegetation in rainforests acts as a significant carbon sink, as well. When they die, that carbon will be released, and the types of plants that grow up when rainforests die won't be able to store as much carbon in the future. Those rainforests that do survive will also be less able to hold onto carbon, according to researchers.

Forest Fires

Forests in mid-latitude places will generally receive less rain and more severe and frequent droughts in the summers, as has already been recorded across the American West and Northwest. These conditions make forest fires spread more quickly over a landscape, as well as more common and hotter (meaning they are more destructive when they burn). When a forest burns, it releases most of the stored carbon that's held in the trees and vegetation, so forest fires are part of the positive feedback loop of increased atmospheric carbon.

Both planned (to clear land for farming) and accidental fires in the Amazon rainforest have similar positive feedbacks for climate change as drier forests do.

Desertification

In drier places, previously forested or vegetation-covered landscapes have converted to or will become desert due to the effects of hotter, drier climate conditions. Over half of the land on the continent of Africa is in danger of desertification, but it affects land on every continent. Desert soils support fewer plants, which keep and use carbon, and have less humus, the part of soil that traps more carbon.

Ice

Ice, and especially glacial ice, reflects back a significant amount of solar energy. So when it melts, the land or water beneath it is revealed, both of which are darker. Darker colors absorb, rather than reflect solar energy, leading to warming. That warming causes more melting, both locally and in the whole climate system.

Other feedback loops take place within this system, like ice melting contributing to sea-level rise, which in turn melts more ice more quickly, so this melting is accelerated. The opposite thing happens during global cooling episodes, with ice building up relatively quickly as the reverse system reinforces itself.

Water Vapor

Water vapor is the most abundant greenhouse gas. How much water vapor can be held in the air is determined by temperature. The warmer the temperature, the more water is able to be held aloft due to the chemistry of the water molecules. So the warmer it is, the more water vapor in the air, which then contributes to further warming.

Below are examples of negative feedbacks.

Clouds

Changing temperatures are expected to change cloud cover, type, and distribution. Since clouds have both a negative and a positive feedback effect, they could be included in both categories, and different scientific research points to different impacts from clouds. But overall, their impacts could be negative, due to the fact that cloud cover reflects sunlight back out into space, creating a cooling effect. Some research has indicated that if CO2 levels triple, all low-lying stratocumulus clouds would disperse, causing significant additional warming.

However, since clouds also trap heat below them, how much of a negative feedback they have is dependent on the height and kind of cloud.

Looking at satellite data from recent years hasn't been a reliable indicator since the data is more useful for snapshots of regions—when extrapolated to planetary cloud cover, the noise in the system renders the information less useful. Modeling is also a challenge with clouds due to the complicated physics involved.

Blackbody Radiation (the Planck Feedback)

The Planck feedback is a very basic part of climate feedback models and is taken into consideration when writing climate sensitivity feedback equations. When features on the planet's surface absorb the sun's energy, their temperature increases and increases the temperature of the surfaces and air around them — a positive feedback. However, not all the energy absorbed is retained at the planet's surface; in this case, it has the effect of increasing how much heat eventually makes its way back out into space. Technically, this is a negative feedback.

Plant and Tree Growth

As the planet warms and gets wetter in many places, more plants will grow and grow more quickly. While they are doing so, they will pull CO2 out of the atmosphere; some of that CO2 will come out in plant respiration over time, while some of it will get buried and stored in the soil. However, there's a limit to this idea; plant growth is limited by other chemicals, especially nitrogen, and the overall effects of climate change (droughts and heat stress among them) mean that plants, in many places, will not be able to survive or thrive in areas where they historically have.

Geological Weathering

As a basic part of the Earth's carbon cycle, chemical weathering of rocks removes CO2 from the atmosphere. The warmer it is and the more it rains, the faster this cycle occurs. Overall, this is a relatively slow process, compared with the ice and water vapor positive feedbacks, but could help mitigate some of the additional CO2 that humans release into the atmosphere.

Primary Measures of Climate Sensitivity

Climate scientists have three main ways to measure climate sensitivity, so if you're analyzing equations, reading journal articles, or perhaps hearing climate scientists discus climate sensitivity, you will hear the following terms used:

Equilibrium Climate Sensitivity

When CO2 levels change, it doesn't affect the global climate immediately. Due to all the various feedback loops and competing factors, the climate takes time to adjust to a rise in CO2 — or reach equilibrium, hence the name equilibrium climate sensitivity (ECS).

To understand this, think about how long it takes for carbon that's stored in a cut-down tree to be released: If the tree is chopped up and used for firewood, it releases that carbon, but it may take 3-4 years before all that wood is burned. Another example is the ocean: it will take many years for the deepest parts of the Pacific to warm a degree—even though that warming will happen, the timescale is very long.

Transient Climate Response

Transient climate response (TCR) is the more immediate warming that occurs when CO2 doubles. This happens before ECS, and is a temporary measure, since additional warming will be known to be coming.

Earth Systems Sensitivity

Earth systems sensitivity looks at even longer-term changes than ECS does. This measure takes into account changes on the scale of multiple decades or more, like glaciers moving or disappearing, movement or disappearance of forest cover, or desertification's effects.

What Happens if CO2 Emissions Are Not Reduced?

If CO2 emissions are not reduced, the climate sensitivity calculations indicate that temperatures will increase globally. That change in average temperature won't be evenly distributed around the globe. In some places, like the Arctic regions, temperatures have increased at twice the rate of other areas. As temperatures continue to rise, more glaciers, ice, and permafrost will melt, accelerating and reinforcing their positive feedbacks with climate change.

We are already seeing the effects of climate change on our world: More frequent and more destructive hurricanes and other storms, drier conditions setting the stage for hotter and more damaging wildfires, an increase in floods, including those associated with sea-level rise which affects the water table in coastal locations and many other impacts. These effects we are seeing today were all predicted in the 1990s.

Environmental Impact

The environmental impacts of climate change are diverse and complex. While there are still many unknowns, we are already experiencing many of the most commonly predicted effects: more extreme storms, more frequent and intense flooding events, sea-level rise, hotter-burning wildfires, and accelerated desertification.

But climate change has less immediately devastating and obvious impacts on the environment in addition to the larger-scale impacts.

Animals

Animals that have specific ecological niches will struggle as those niches rapidly change or move due to climate change. This will affect a range of animals, including, but not limited to:

• those that depend on snow or ice cover, like polar bears or Canada lynx;
• those that are only able to survive in specific water temperatures like coral and fish;
• and those who rely on seasonal water, known as ephemeral pools, including a range of insects and amphibians.

Other animals will be impacted by their food sources moving or disappearing, which has a profound impact on survival. Songbirds are already adjusting their migration routes to deal with climate-changed landscapes, in some cases having to fly further for food or water, as well as deal with more extreme weather events and wildfires, which is presumed to be behind recent unprecedented mass die-off events.

Plants

The distribution and abundance of plants will be affected by climate change on multiple levels. In areas affected by drought, some plants won't have enough water to grow and reproduces. Others, like the iconic Joshua Tree, won't be able to adapt quickly enough to changing conditions.

Human Impact

A more volatile and destructive weather system has tremendous impact on human lives and activities. Those people with fewer resources to move or rebuild will suffer at much greater rates than those people in richer countries or who have personal wealth. That means that the majority of the negative effects of climate change — loss of life, as well as homes, businesses, and basic resources like clean water — have already and will continue to be borne by those with the least.

This holds true even within countries with higher per-capita incomes. For example, the Fourth National Climate Assessment, a joint publication by various U.S. agencies including NOAA, found that poorer people and communities in the US will suffer disproportionately from climate change impacts.

Economics

Climate change effects will also be costly. Estimates of the costs of climate change vary depending on what is included: Some studies look at the costs of increasing disasters on global trade alone, while others look at the cost of disruption to "free" ecosystem services—the work that a wetland does in filtering water, for example.

Climate sensitivity currently has a wide range: that 2 to 4.5 degrees of global temperature rise that is predicted will come with a doubling of CO2 levels. Just the uncertainty of how severe the temperature rise will be is estimated at $10 trillion dollars, according to a study from the University of Cambridge.

Human Life

People will die earlier than they would have otherwise due to climate change effects. Indigenous communities will be less able to hunt, gather, and engage in traditional practices in ecosystems that are unable to support the plants and animals traditionally found there.

We are already past the time when making more significant reductions in CO2 could avoid significant warming.