Advanced climate modelling for policymakers

Earth system feedbacks can significantly alter the degree of warming caused by greenhouse gases

The new Met Office model increases our confidence that reducing greenhouse gas emissions to 50% of current levels by 2050 would make it possible to meet a 2 °C global warming limit.

Earth system models are climate models that include key elements of the biosphere (biology of the land surface and ocean), and atmospheric and ocean chemistry. Many of these Earth system components are themselves affected by climate change and air pollution.

These so-called 'Earth system feedbacks' can significantly alter the degree of warming caused by human emissions of greenhouse gases — or, alternatively, the extent to which we need to reduce greenhouse gas emissions in order to achieve a particular temperature target.

Advance - improved science Advance — improved science for mitigation policy advice (PDF, 1 MB)

Our new model

HadGEM2-ES is the latest Earth system model to be used at the Met Office Hadley Centre and is one of the world's most complex and sophisticated. It simulates many more potentially important Earth system processes than earlier models, allowing us to consolidate our understanding of how climate works and improve the quality of our climate projections.

Key processes in the new model

Land carbon uptake

The higher the concentration of CO2, the more is absorbed by plants. Climate change may lead to the expansion of boreal forests at high latitudes, but cause the loss of rainforests in the hotter, drier conditions of the Tropics.

Ocean carbon uptake

CO2 dissolves in seawater. The amount in the surface waters is controlled by circulation, which moves CO2 into the deep ocean. Surface warming can lead to a weaker circulation, slowing CO2 removal from the surface & the atmosphere.

Methane (CH4)

Emissions from wetlands accelerate rapidly in warmer conditions, but may be reduced if conditions become drier.

Lifetime of CH4

The amount of time CH4 remains in the atmosphere depends on how quickly it's oxidised. A warmer, moister atmosphere leads to more rapid loss.

Ozone

Important greenhouse gas affected by chemical reactions involving other pollutants. It also damages plants. We are currently working towards including this fully in the model.

Windblown atmospheric dust

Aerosol strongly affected by changes in soil moisture, wind & vegetation cover. It also nourishes some ocean plankton species, affecting the ocean carbon cycle.

Ocean plankton

Some ocean plankton species emit dimethyl sulphide, which can form sulphate aerosols, contributing to a cooling climate. This process is sensitive to changes in ocean conditions.

Black carbon

Soot from combustion absorbs sunlight, warming the climate. The deposition of black carbon on snow changes the reflectivity of the surface, which then causes more warming at high latitudes. We are currently working towards including this fully in the model.

Pathways

The new Earth system model can be used to project future climate change for a given pathway of greenhouse gas concentrations, aerosol emissions and land-use changes. We're also using it to estimate what level of human CO2 emissions could be permitted that are consistent with this pathway.

A new type of pathway is being used for the next Assessment Report from the Intergovernmental Panel on Climate Change in 2013 — the Representative Concentration Pathway or RCP. These represent very different views of how the world may look in 2100, with RCP 2.6 showing the effects of strong mitigation and RCP 8.5 the impacts of 'business as usual' in which we continue to use fossil fuels with no mitigation.

RCP 2.6

Although the mean temperature rise under RCP 2.6 is between 1.5 and 2 °C by the end of the century, many regions will experience much greater (or lower) increases in temperature. For instance, Arctic increases of about 8 °C are possible in 2100 for this scenario. The average warming for land regions is 2.3 °C, compared to 1.8 °C for the global average.

RCP 8.5

If emissions continue to rise, leading to the RCP 8.5 scenario, then the global temperature is predicted to reach 5.6 °C above the pre-industrial level by 2100 and is still rising by 0.45 °C per decade at the end of the century. Some regions are projected to warm by more than 15 °C (Arctic). The impacts of such a scenario are likely to be large and costly.

Findings

Our simulations show that a peak in greenhouse gas emissions in the first decades of the 21st century, followed by 50% cuts in emissions by 2050, are compatible with a 2 °C global warming limit (RCP 2.6). This backs up results from simpler models and considerably increases our confidence in this key conclusion.

Even so, it's important to bear in mind that these results come from just one model. Some additional processes still need to be included, and the results need to be compared with similar models as they are developed around the world.

What next?

Thanks to the latest generation of Met Office climate models, we already understand more about how man-made emissions of greenhouse gases are linked to global and regional temperature during the 21st century. The latest models allow us to add more information about complex interactions involving different pollutants — critical for answering specific policy questions.

Given their major implications for international technology and economic development, policy decisions on climate change must be underpinned by the best possible evidence. We will continue to improve the representation of processes included in our model that could potentially have a major impact on the degree of warming for a given emissions scenario, quite apart from their impact on local and regional climate.

Other processes that are actively being looked at, with a view to including them in future models include:

  • ability of plants to take up nitrogen, which may be limited by the supply of nitrogen naturally and enhanced by man-made sources of nitrogen.

  • deposition of black carbon in very cold regions;

  • the positive and negative feedbacks of ozone;

  • thawing of permafrost, which may lead to large amounts of carbon release (but these processes are not well understood).

  • dynamic ice processes that could speed up the supply of freshwater from glaciers into the ocean.

  • processes that affect methane in the Arctic Ocean, which could lead to increased methane release (but, again, the science is not well understood).

Understanding the interactions within the Earth system will continue to be essential for mitigation policy advice.

Case studies

Black carbon aerosol

Black carbon (BC) is a type of atmospheric aerosol made up of small particles that exist in the atmosphere from natural and man-made sources and varying in chemical composition (e.g. dust, sea-salt, smoke, sulphate). It has a potent warming effect.

BC aerosols are very efficient at absorbing sunlight, which heats the atmosphere. BC can also change cloud properties and cloud cover; and, if deposited in very cold regions, can darken ice and snow surfaces increasing the absorption of sunlight.

Different studies suggest that BC climate forcing causes regional circulation and precipitation changes. These include a northward shift in the Intertropical Convergence Zone (where the strongest tropical convective rainfall occurs near the Equator) and changes in the Asian monsoon systems where concentrations of absorbing aerosols are large.

Science can inform the policy discussion around the role of BC in the mix of greenhouse and other radiatively active gases. Due to its short lifetime in the atmosphere, reducing BC emissions has the potential to reduce the rate of climate change in the short-term. For this reason, directly reducing BC as a way of mitigating climate change is now receiving policy attention.

Importance of ozone

Ozone is a greenhouse gas which, higher up in the atmosphere, has a very important role. It absorbs energy from the Sun producing a warm, stable stratosphere and protects the surface from harmful ultraviolet radiation. Changes in upper-level ozone — in particular, 'the ozone hole' — have been linked to man-made pollutants and the Montreal Protocol, agreed in the late 1980s, was put in place to protect upper-level ozone.

Ground-level ozone is formed in the lower atmosphere through the chemical reactions of pollutants, involving hydrocarbons and oxides of nitrogen (NOX). Fossil-fuel burning is a significant source of these pollutants as well as CO2 which explains why human activity at least doubled the concentrations of ground-level ozone over the 20th century.

At ground level, ozone damages plants so has major implications for the ability of plants to take up CO2. Lower plant productivity in polluted air means less carbon is stored in the plants and soil, so more CO2 remains in the atmosphere. The climate change caused by this extra CO2 can be as important as the greenhouse impact of ozone itself.

In the lower levels of the atmosphere, high concentrations of ozone are poisonous to people and animals as well as plants. Air quality controls that limit ground-level ozone therefore have a double benefit: on health and on climate — something that is now being recognised by policymakers.

Last updated: 18 September 2013