What you need to know about climate sensitivity
Climate sensitivity is a subject sometimes explored in mainstream media articles. For example, The Economist tried to summarize some recent research on the subject, although as climate scientist Michael Mann and I noted in an article for ABC , they made some key mistakes. We know that humans have increased the greenhouse effect due to the carbon emissions associated with burning fossil fuels. This increased carbon dioxide traps more heat in the Earth's atmosphere, causing a global energy imbalance. There is more energy incoming than escaping, and as a result the planet will warm until it reaches a new balanced energy state (equilibrium), with equal incoming and outgoing energy. We also know that if we double the amount of carbon dioxide in the atmosphere, the increased greenhouse effect will cause the planet's average surface temperature to warm about 1.2C (2.2F) in response. That may not sound like very much, but the difference between an ice age and the current warm period is only about 5C (9F). Seemingly small temperature changes make a big difference in the Earth's climate. In addition, there are feedbacks that can dampen or amplify the warming from the increased greenhouse effect. For example, when ice melts it makes the Earth's surface less reflective, causing it to absorb more sunlight and warm further. A warmer atmosphere will also hold more water vapor, and water vapor is another greenhouse gas. The term "equilibrium climate sensitivity" refers to the total amount of warming that will occur at the Earth's surface once it reaches a new balanced energy state, including from the increased greenhouse effect from a doubling of atmospheric carbon dioxide, and including these feedback effects. Water vapor is probably the largest individual feedback. A 2009 study published in the prestigious journal Science by Andrew Dessler and Steven Sherwood found that, as climate scientists expected, the amount of water vapor in the atmosphere is increasing by enough to double the warming from the increased greenhouse effect of carbon dioxide. As discussed above, melting ice is another significant warming feedback. Releases of stored carbon from beneath permafrost and methane from the deep ocean would also amplify global warming. Unfortunately we don't know of many large negative feedbacks that would dampen global warming. Global plant growth has increased so far, and plants absorb carbon dioxide. But that trend probably won't last as extreme weather events like heat waves and droughts that damage plant life become more common. Clouds are really the only plausible feedback that could significantly dampen future global warming. They're tricky because clouds cause both warming by increasing the greenhouse effect, and cooling by reflecting sunlight away from the Earth's surface. High clouds tend to have an overall warming effect, while low clouds tend to have an overall cooling effect. So what types of clouds will become more abundant in a warming world, and what will the net effect on temperatures be? We don't yet know the answer to that question. Thus climate contrarians often argue that clouds will save us from dangerous climate change. However, the evidence so far from researchers like Andrew Dessler indicates that clouds don't play a large role in either amplifying or dampening global warming, at least in the short-term. There are three main categories of studies estimating climate sensitivity, which are based on: 1) very detailed climate models. 2) combining recent climate measurements with simpler climate models. 3) measurements of past climate changes. Most studies have been very consistent in estimating that surface temperatures will warm between 2 and 4.5C (3.6 to 8.1F) in response to a doubling of atmospheric carbon dioxide, most likely 3C (5.4F). As The Economist article noted, there have been a few recent studies estimating climate sensitivity toward the low end of that range, mainly using the second approach listed above, applying recent measurements to simple climate models. However, as Mann and I noted in our ABC article, that approach is subject to large uncertainties that could cause it to either underestimate or overestimate the true climate sensitivity. Meanwhile, a major recent study published in the journal Nature examined nearly two dozen studies estimating climate sensitivity based on past climate changes over the past 65 million years. These were all consistent with the established range of climate sensitivity estimates between about 2 and 4.5C. Estimates based on detailed climate models are also consistent with this range. Every approach and each individual climate sensitivity study has its own drawbacks. Past climate data tells us how the climate previously changed, but the future Earth will not be identical to the Earth of millions of years ago. Climate models are very detailed simulations of the Earth's climate, but they are still just simulations. There are significant uncertainties associated with the second approach listed above, and it also relies on climate model simulations. However, when we put all the evidence together, we can be confident that average surface temperatures will warm between about 2 and 4.5C in response to a doubling of atmospheric carbon dioxide. It's also important to remember that this range is based on a large body of evidence using several different approaches, which all give us about the same answer. No single study is going to overturn that vast body of evidence. So far we're about 40% of the way to doubling atmospheric carbon dioxide compared to levels before the Industrial Revolution, and rising fast. If we continue on our current track, we'll reach the doubled carbon dioxide mark around mid-century; even sooner when the warming from other human greenhouse gas emissions like methane is included. That's bad news, because it's internationally accepted that more than 2C surface warming would be very dangerous. So we're on pace to commit ourselves to 2 to 4.5C warming by mid-century, and 2C is considered too dangerous: the math doesn't look good. It tells us that even if climate contrarians are right and climate sensitivity is relatively low, our current emissions path is still unacceptably dangerous . So while there is still a lot of uncertainty about the Earth's exact climate sensitivity, and while it's an important question to resolve, from a policy standpoint, it really doesn't matter. Whether climate sensitivity is 2C or 3C or 4.5C, we're not doing enough to avoid very dangerous climate change in any case. From a policy standpoint, climate sensitivity will only be a relevant issue once we start to take serious steps to reduce carbon emissions from fossil fuels.