How much does the world warm up in response to a certain amount of greenhouse gases like CO2 in the atmosphere? It’s a simple question, but its answer depends on whether you mean short-term or long-term warming, and estimates vary according to the methods used. Scientists are currently intensively debating long-term ‘climate sensitivity’, which
begs prompts the question: might we be pushing too hard to cut climate CO2 emissions, if this is uncertain?
The answer is no, according to Joeri Rogelj from the Swiss Federal Institute of Technology, ETH Zurich, and his coworkers. They looked at how a range of climate sensitivity values affected their 21st century warming projections in a paper published in Environmental Research Letters last week. “When taking into account all available evidence, the big picture doesn’t change,” Joeri told me. The ‘carbon budget’ of greenhouse gases we could still emit today and in the future is very limited whatever the climate sensitivity, he explained. “Keeping the so-called carbon budget in line with warming below 2°C still requires a decarbonisation of global society over the first half of this century.”
Climate sensitivity is the measure of how much the world will eventually warm when it reaches equilibrium after a doubling of CO2 in the air. Today, we have upset the normal equilibrium where the Sun’s energy flowing into the atmosphere matches the flow the Earth radiates back out of it. Now more is coming in than leaving, and that’s heating the planet up. Think of the atmosphere as a series of pipes, with energy flowing through them like a liquid. The Earth is a reservoir in the system, filled by an incoming pipe and drained by an outgoing one. CO2 acts like a blockage in the outgoing pipe – it slows the outward energy flow and causes a build-up in the reservoir. When the reservoir gets fuller it can put enough pressure on the blockage for the outward flow through it to again match the incoming flow. Then we’d be at equilibrium, but with a fuller reservoir – a warmer planet. The more CO2 we emit, the worse the blockage gets and the hotter we get before reaching equilibrium.
Not so sensitive
The UN Intergovernmental Panel on Climate Change (IPCC) says that the warming at equilibrium after doubling CO2 would likely – more than a two-in-three chance – be between 1.5 and 4.5°C. It’s extremely likely – more than a nineteen-in-twenty chance – that it would be above 1.5°C. The most likely value seems to be near 3°C, but several recent studies have put it either at the top or bottom of the usual range. And though this may sound like a dry, academic, argument, these numbers are regularly used as ammunition in the political conflicts over climate change. “Each time a new climate sensitivity estimate is published, statements are made about its policy implications,” Joeri underlined.
One of Joeri’s specialities is using climate models to project how our CO2 emissions influence the odds of meeting targets for limiting global warming. He’d previously been in a team who found that to stay under the 2°C threshold governments agreed in Copenhagen in 2009 emissions would have to peak by 2020. Being aware of the ongoing debate, in their main study Joeri and his coworkers therefore fed four climate sensitivity estimates into their existing models. They had previously used estimates from the IPCC’s 2007 report that gave an average figure of 3.0°C, and added its newly-released changes, which remain very similar. The other two estimates they used had average climate sensitivities of 1.9°C and 3.9°C.
Using all these estimates in each of four different scenarios ranging from low to high CO2 emissions, the temperatures Joeri’s team projected by 2100 look surprising at first glance. The greatest sensitivity increased temperatures by around one sixth, and the lowest reduced them by about a quarter. But these more extreme climate sensitivities are one-third higher or lower than the IPCC estimates, respectively. Why aren’t the temperatures changing by a third too? It’s related to the earlier definition of equilibrium. Not enough time has passed for the reservoir-filling to have finished, so the CO2 blockage’s full warming impact hasn’t yet been felt.
That delayed impact on warming feeds through to limit the effects on the targets we need to set. In the team’s lowest emission scenario high climate sensitivity would cut the chances of keeping warming below 2 °C from 81% to 72%. Low sensitivity would improve the chances to a 98% likelihood we’d meet the target in that scenario. However, if our emissions stay on their current path low sensitivity would delay passing the 2 °C threshold by less than a decade. “There is no scientific support to diminish the urgency of emission reductions if warming is to be kept below 1.5 or 2°C, the two temperature limits currently being discussed within the United Nations,” the scientists write.
One reason optimistically avoiding tackling greenhouse gas emissions is dangerous is because continuing to build fossil fuel power stations would commit us to using them. Once the CO2 they emit is in the air it will be even more costly to get back towards staying within temperature limits, Joeri warned. “Only betting on the results from studies with lower estimates would neglect the information we have from other studies that end up at the higher end,” he added. “At this point, we are not able to say which of these methods will turn out to be superior in predicting the actual climate sensitivity value.”
Rogelj, J., Meinshausen, M., Sedláček, J., & Knutti, R. (2014). Implications of potentially lower climate sensitivity on climate projections and policy Environmental Research Letters, 9 (3) DOI: 10.1088/1748-9326/9/3/031003