27 February, 2020

New models, old message: deep emission cuts in next decade are key

Mkjr 7 Ku2 C1 Uo Xe U unsplash

A number of the latest generation climate models (CMIP6) project greater future warming than previously assessed, but drawing conclusions about the implications for emission reduction targets is premature.

The higher warming these new models project is due to higher climate sensitivity, which might be partly explained by how these models incorporate new knowledge about the interactions between clouds and the climate.

A growing number of studies suggest that CMIP6 model sensitivities might lead to overestimating future warming. More importantly however, the change in the quantity in CMIP6 models, which is relevant for calculating carbon budgets - the transient climate response to emissions (TCRE) - may be smaller than the potential increase in long-term equilibrium warming.

Preliminary results based on a limited set of models suggest that even TCRE estimates from more sensitive CMIP6 models, if proven correct, would only reduce the best estimate for the 1.5°C carbon budget by a few percent. These differences are very small compared with the uncertainties surrounding these carbon budget estimates, and do not allow for any robust conclusions. If anything they only re-emphasise the need for stringent near-term emission reductions to achieve the Paris Agreement goal.

The world is 1°C warmer than pre-industrial levels and continues to warm at about 0.2°C per decade. Slowing down warming over the next decades is crucial to limit warming to 1.5°C. Stringent emission reductions, as implied by Paris Agreement compatible pathways, can reduce near-term warming rates by up to 50%. Near-term emissions reductions are key to keep the Paris Agreement’s long-term temperature goal in sight.

The Intergovernmental Panel on Climate Change (IPCC), the United Nations body for assessing the latest climate change science, will release its 6th Assessment Report (AR6) in 2021. A great deal of the findings in AR6 will be based on the latest generation of climate models from the 6th phase of the Coupled Model Intercomparison Project (CMIP6). Therefore, it is crucial to assess whether these models provide a better representation of the climate system and offer more realistic projections of global warming than their predecessors.