The science behind climate change

Climate science is the study of regional and global climate as a system. While meteorologists try to predict forthcoming local weather using detailed analysis of recent patterns, climate scientists aim to understand, and replicate through modelling, how the climate system works as a whole.

Climate scientists use a vast array of monitoring data to create detailed supercomputer models that reproduce the interacting mechanisms of the climate system in ever-increasing detail. As we learn more, the modelling becomes ever more complex and the output more accurate. We can look far into the future or focus on local effects, and experiment with the many different variables that influence climate.

We want to understand how our climate is changing, how variations in the contributing systems affect each other and the likely impact of these changes on ecosystems and human life - on a global, regional and local scale.

Among other things, our climate models are predicting how increasing carbon dioxide (CO₂) levels will affect global temperatures and what reduction in carbon dioxide CO₂ levels is necessary to keep global warming in check.

Emissions pathways

Our climate scientists have been working with the UK government and other research agencies on a programme called The AVOID2 programme on avoiding 'dangerous' climate change, which explores the potential long-term effects of 'dangerous' climate change. We define 'dangerous' climate change effects as those that are irreversible, spiraling (self-reinforcing), very large or very rapid, and with a serious impact on natural and human systems.

If global emissions continue unabated at their current rate, the planet's surface temperature is projected to reach 4 °C above pre-industrial levels by the close of the current century. That is the 'business as usual' pathway, and leads to dangerous climate change on a number of fronts. The AVOID programme examined emissions pathways that would limit warming, the mitigation measures that would make it possible and the impacts of climate change that could be avoided.

The ultimate objective of the United Nations Framework Convention on Climate Change (UNFCCC) is to stabilise atmospheric greenhouse gas concentrations at a level that would prevent dangerous interference with the climate system. Stabilising greenhouse gas concentrations in the atmosphere at sufficiently low levels will limit global temperatures rise to under 2 °C until at least 2500.

Dangerous changes to the climate system

• Melting sea ice: although sea ice melting may be reversible, the damage it causes won't be.
• Sea level rise: any rise will cause severe and irreversible harm to coastal habitats.
• North Atlantic drift shutdown: severe warming could shut down the Atlantic meridional overturning circulation, which warms the British Isles and north-west Europe, leading to a temperature drop.
• Tropical forest fires: drier tropical forests may escalate the loss of CO₂-absorbing biomass.
• Emissions from wetlands, permafrost and oceans: increased temperatures will release more organic greenhouse gases, potentially in a vicious circle.
• Extreme temperatures: heatwave summers could become very common.

Impacts on human and natural systems

• Coasts: the cost of sea level rises could amount to hundreds of billions of dollars each year.
• Oceans: increased acidity will further harm coral reef systems.
• Ecosystems and biodiversity: tropical ecosystems have joined polar, mountain, coral and Mediterranean on the list of the most vulnerable habitats, which will be severely damaged by 2-3 °C warming.
• Fresh water: big changes in rainfall patterns will severely reduce runoff in some areas and increase it in others, leading to raised flood risk. The proportion of land in extreme drought could rise from 1% to almost a third by the end of the century.
• Agriculture and food security: crop yields are expected to decline with climate change.
• Human health: with 2 °C of global warming, cold-related deaths would decline but heat-related deaths would increase. Studies suggest cases of malaria, too, would increase.

Feasibility of a 2 °C limit

Modelling results from the Met Office Hadley Centre performed for the Committee on Climate Change developed a feasible idealised emission scenarios leading to a 50% probability of limiting warming to 2 °C . This scenario requires a peak of emissions in 2016 or earlier, followed by a 4% rate of global emissions reduction.

Limiting the warming will require local, national and international co-operation, and a range of renewable fuels and carbon capture technologies to be fully developed, assessed and implemented.

Emissions reduction, or mitigation, will be costly. However, the cost of damage to human and natural systems if global warming is not kept below 2 °C is many times greater.