Global temperatures are rising due to accumulation of greenhouse gases in the atmosphere. In addition to increasing greenhouse gas emissions, human activity has also changed the composition of particles in the atmosphere. These aerosols have masked some of the warming caused by the greenhouse gases. How large is this cooling effect, and how has it changed over time?
In the project Understanding Temporal aerosol Radiative forcing - Implications for Climate Sensitivity and future warming the temporal development of the cooling effect of aerosols is studied to improve estimates of climate sensitivity and hence future warming.
Climate sensitivity is a key concept in climate research that represents the temperature rise for a given increase in CO2 concentration. Estimates of climate sensitivity can be made either by using complex climate models or historical observations of how the climate has changed over time in combination with estimates of the historical climate forcing. Both methods have limitations. In climate models the representations of clouds and how they change in a warmer climate, are uncertain. For the second method, which is used in this project, the uncertainty lies in the magnitude of the historical climate forcing, mainly due to the uncertain effect of aerosol cooling.
The project group has previously published estimates of climate sensitivity and showed in 2018 that the time development for aerosol cooling was crucial for the climate sensitivity estimate. In this project, uncertainty in the time development will be implemented in the method for estimating climate sensitivity. The uncertainty in the time development of aerosol radiative forcing will be quantified, taking advantage of international multi-model studies, long term observations and uncertainty in historical emission inventories. Aerosol precursor emissions show a rapid decrease over the last decade in the most recent emission inventory. We started to focus on this period as emission estimates used in the multi-model studies do not include the most recent emissions, and the period coincides with rapidly increasing global mean temperature.
With a better estimate of aerosols radiative forcing and climate sensitivity in combination with scenario data, we will illustrate the uncertainties in global temperature change over the coming decades. This is essential for the goal in the Paris Agreement to limit warming to below 2 or 1.5°C.
The large uncertainty in the climate sensitivity is a key topic in climate research and vital for our understanding of the severity of global warming. The complex feedback processes in the climate system make it difficult to determine how sensitive the Earth is to increases in greenhouse gases, and thus how stringent mitigation measures are needed to keep warming below 2°C or even 1.5°C.
To improve the knowledge of future climate change, we must increase our understanding of past drivers of climate change. Historically, anthropogenic aerosols have masked part of the warming due to the increase in greenhouse gases. The UTRICS project investigates the temporal development of the aerosol effective radiative forcing (ERF). Results of detailed aerosol modeling, taking advantages of international multi-model initiatives, analysis of aerosol observations, as well as in-house simulations, will be combined with observational based time series of global temperature and ocean heat content using an established method, to infer estimates of aerosol ERF time series and climate sensitivity. Our method is a necessary complement to earth system models, both with respect to the highly parameterized processes related to the climate feedbacks and to aerosol and aerosol cloud interaction.
What implications do estimated climate sensitivity and forcing have on society? This will be illustrated within UTRICS by using the results and future scenarios to calculate the anthropogenic warming over the next 2-4 decades for different levels of aerosol ERFs. This will indicate by how much the uncertainties in projected anthropogenic warming can be reduced if we gain improved knowledge of the aerosol ERF.
UTRICS represents the next step in observational based estimates of climate sensitivity, by taking into account the uncertainties in aerosol forcing time development.