No shortcuts to calculating temperature rises in climate models

Scientists consider equilibrium climate sensitivity (ECS) an essential number for comparing climate models. A measure of how much Earth’s surface will warm following a doubling of CO2 in the atmosphere, ECS has for decades been predicted to range between 1.5 ℃ and 4.5 ℃. A new study carried out as part of the EU-funded TiPES project now reveals that it could still be quite difficult to accurately estimate ECS in complex climate models.According to the study’s authors, the limited data and relatively brief simulations required to estimate ECS could be significantly underestimating long-term warming. This is because commonly used methods assume that climate response, in other words the change in the global mean surface temperature, is linear. However, in reality, response can be non-linear, sometimes becoming visible immediately after a large perturbation and at other times only becoming apparent after a long period.

The complexity of Earth’s climate system means that it can take thousands of years to reach an equilibrium temperature. However, climate model simulations run on today’s supercomputers take a few months to yield results on 150 years of climate change. Therefore, to simulate the thousands of years of climate change in order to find a climate model’s ECS, we would have to run the model for years on end.

Since this is not feasible, scientists have used an easier method explained in a news item posted on ‘AZoCleantech’. “Once a model has been subjected to the simulation of a couple of hundred years of climate evolution, the data is gathered and then utilized to evaluate how much the mean global temperature goes up if the model was allowed to run until the equilibrium temperature was reached.”

However, this simplified approach – based on calculations rather than simulations – could be underestimating the rise in surface temperature. This is because calculations do not take into account how late climate tipping events, for example, a sudden desertification after thousands of years, can abruptly change mean global temperatures.

Therefore, climate tipping makes the standard approach to comparing climate models unreliable. Since this method has the potential to fail in simple climate models, it will likely also be inadequate for more advanced models.

“So, we show that to be sure of the long-term behavior of a modern global climate model, there are no shortcuts to doing extensive simulations,” remark study authors Drs Robbin Bastiaansen and Anna von der Heydt of TiPES (Tipping Points in the Earth System) project partner Utrecht University, the Netherlands. “If you want to know what ultimately is the response/temperature for a given amount of added CO2, there will be no easy, straightforward and sound way to determine that for sure – even in models.”

For more information, please see:

TiPES project website