University of Exeter Peter Cox and his fellow researchers have a new study, published in leading scientific journal Nature, which reduce the range of possible temperature increases by around 60 percent.
They found that the widely variable range of 1.5 – 4.5 ºC (2.7 – 8.1 ºF) promoted by the U.N.’s Intergovernmental Panel on Climate Change (IPCC) is a much narrower change ranges of 2.2 – 3.4 ºC (4 – 6.1 ºF). The study’s best estimate is that global temperatures will change by 2.8 ºC (5 ºF) by 2100.
Equilibrium climate sensitivity (ECS) remains one of the most important unknowns in climate change science. ECS is defined as the global mean warming that would occur if the atmospheric carbon dioxide (CO2) concentration were instantly doubled and the climate were then brought to equilibrium with that new level of CO2. Despite its rather idealized definition, ECS has continuing relevance for international climate change agreements, which are often framed in terms of stabilization of global warming relative to the pre-industrial climate. However, the ‘likely’ range of ECS as stated by the Intergovernmental Panel on Climate Change (IPCC) has remained at 1.5–4.5 degrees Celsius for more than 25 years1. The possibility of a value of ECS towards the upper end of this range reduces the feasibility of avoiding 2 degrees Celsius of global warming, as required by the Paris Agreement. Here we present a new emergent constraint on ECS that yields a central estimate of 2.8 degrees Celsius with 66 per cent confidence limits (equivalent to the IPCC ‘likely’ range) of 2.2–3.4 degrees Celsius. Our approach is to focus on the variability of temperature about long-term historical warming, rather than on the warming trend itself. We use an ensemble of climate models to define an emergent relationship2 between ECS and a theoretically informed metric of global temperature variability. This metric of variability can also be calculated from observational records of global warming3, which enables tighter constraints to be placed on ECS, reducing the probability of ECS being less than 1.5 degrees Celsius to less than 3 per cent, and the probability of ECS exceeding 4.5 degrees Celsius to less than 1 per cent.
Editorial Summary – Narrowing down long-term global warming estimates
Equilibrium climate sensitivity (ECS) is the long-term change in global mean surface temperature predicted to occur in response to an instantaneous doubling of atmospheric carbon dioxide concentrations. It is an inherently artificial metric, but is nonetheless an important tool when comparing climate models, and a key point of policy discussion. The seemingly intractable range of ECS estimates complicates policy making because the response of the real climate system to the lowest and highest predicted temperature change would translate into radically different policy options. Peter Cox and colleagues now constrain climate models by their ability to simulate observed variations in climate, and conclude that ECS has a central estimate of 2.8 degrees Celsius (°C), which sits towards the middle to lower end of current estimates, and a range of 2.2–3.4 °C. Importantly, their approach allows them to almost exclude ECS estimates above 4.5 °C or below 1.5 °C. show less
Getting a better handle on tipping points
Lead-author Professor Peter Cox from the University of Exeter said: “You can think of global warming as the stretching of a spring as we hang weights from it, and climate sensitivity as related to the strength of the spring.
“To relate the observed global warming to climate sensitivity you need to know the amount of weight being added to the spring, which climate scientists call the ‘forcing’, and also how quickly the spring responds to added weight. Unfortunately, we know neither of these things very well”.
The new research made their breakthrough by moving their focus away from global warming trends to date, and instead studying variations in yearly global temperatures.
Co-author Professor Chris Huntingford, from the Centre for Ecology and Hydrology, explained: “Much of climate science is about checking for general trends in data and comparing these to climate model outputs, but year-to-year variations can tell us a lot about longer-term changes we can expect in a physical system such as Earth’s climate.”
Mark Williamson, co-author of the study and a postdoctoral researcher at the University of Exeter, carried out the calculations to work-out a measure of temperature fluctuations that reveals climate sensitivity.