Near-term climate forcers

TerraFIRMA investigates the role of four important Near-Term Climate Forcers (NTCFs) in the Earth system and assesses their potential for climate mitigation.

NTFCs have lifetimes ranging from a few hours to a decade, we will look specifically at methane, tropospheric ozone, sulfur dioxide and ammonia. Unlike carbon dioxide, changes in NTFC emissions result in rapid, often regional, impacts on the atmosphere. A major uncertainty in the role of NTCFs lies in an inadequate understanding of their interactions and feedbacks across the coupled Earth system, including how such interactions impact regional scale circulations such as the global monsoons. We are assessing these interactions, and how changes in the emissions or burden of one NTCF impacts the lifetime of others and how these changes translate into forcing of regional and global climate.

Credit:  Shibasish Saha / Climate Visuals Countdown

Methane is currently growing at an alarming rate. The cause of this growth remains unresolved and involves changes in both human and natural its sources (e.g. fossil fuels, agriculture, wetlands, wildfires) and (or) sinks (e.g. chemistry, and soil uptake). Changes in climate have a strong feedback on methane emissions from wetlands, which are particularly large and uncertain, so understanding this response is vital. TerraFIRMA will use the methane emission driven UKESM2 model with interactive wetland emissions and isotopes, combined with Earth Observation (EO) data, Machine Learning (ML) and Data Assimilation (DA) to explore the drivers and feedbacks involving methane under a range of future scenarios and to understand the role of variable sources and sinks in the recent rise in methane and future projections.

Ozone is a potent greenhouse gas (GHG) and air pollutant. Studies show it is likely to increase in industrialised areas in the future, but our understanding of historic trends is limited. Stratospheric ozone plays an important role in the tropospheric budget of ozone and is projected to change significantly under climate change. Changes in surface ozone can impact vegetation growth and terrestrial carbon uptake.  Given these interactions there is a need to analyse the role of ozone in the Earth system holistically. TerraFIRMA will use CMIP6 data and new UKESM2 simulations to quantify the role of different sources on trends in both tropospheric and stratospheric ozone. We are investigating uncertainty in these trends, including the role of methane emission trends in setting regional ozone trends with an aim of informing the Gothenburg Protocol.

Aerosols have played an important cooling role in offsetting the warming effect of historic greenhouse gas emissions on climate. Whilst Black Carbon and other aerosols make important contributions, sulfur dioxide emissions have dominated historically. Current emissions of sulfur dioxide are greatest in South and East Asia and strongly impact regional monsoon systems. South Asia is a large source of ammonia and interactions between ammonia and sulfur dioxide, mediated by nitrate aerosol, are a major source of uncertainty in regional aerosol forcing. TerraFIRMA will use UKESM2, with both sulfur and nitrate chemistry and aerosols fully simulated, to reduce uncertainty in the role of sulfur dioxide in climate and assess the role of nitrate aerosol in past and future climate. A major emphasis will be on the role of variable aerosol emissions in forcing past and future changes in regional monsoon circulations.

Mitigation of climate change and poor air quality through NTCF emission reduction is an area of high policy interest. With key stakeholders, TerraFIRMA will develop new mitigation experiments using UKESM2, modifying the magnitude and timing of reductions in NTCF emissions (and their precursors) based around the Shared Socioeconomic Pathway (SSP) framework and addressing, for example, the COP26 methane pledge.