The Earth's climate is always changing. Changes in the Earth's orbit around the sun, alongside complex interactions between volcanic eruptions, the ecosystems on land, the oceans and the atmosphere, ensure that the Earth's climate is always on the move. Far in the past there were Ice Ages and even further back forests in Antarctica.
What is occurring now is unusual: we are experiencing extremely rapid climate change. The air temperature at the Earth's surface increased over the 20th century by 0.7° Celsius and is now rising by 0.2° Celsius per decade. The speed of change is probably faster than at any time in Earth's history, except perhaps after rare events like impacts from large objects colliding with Earth from space.
There has been an enormous scientific effort to explain this temperature rise. Researchers from many branches of science have studied possible changes in the Earth's orbit, changes to the output of energy from the sun, the impact of volcanoes, and other natural factors.
They have also looked at many factors caused by human activities such as emissions of carbon dioxide from burning fossil fuels, changing land-use, the release of small particles called aerosols, and many other industrial pollutants. They found that many processes are contributing to the recorded rise in temperature, but the single most important cause is the increase in the amount of the gas carbon dioxide in the atmosphere. By burning fossil fuels (oil, coal, gas) and forests, humans have increased the amount of carbon dioxide in the atmosphere from about 280 parts per million before the Industrial Revolution to about 380 parts per million today, higher than it has been for at least 650,000 years.
Carbon dioxide is one of a number of ‘greenhouse gases', so- called because they allow the Sun's rays to reach the Earth's surface but then prevent some of that heat escaping back to space, acting a bit like a giant greenhouse around the Earth. This ‘greenhouse effect' has been understood by scientists for over 100 years. It keeps the Earth's surface about 30°C warmer than it would be otherwise.
The increase in carbon dioxide due to fossil fuel use is intensifying the greenhouse effect, heating up the Earth. Because the Earth is a connected and inter-dependent system the increase in temperature is affecting many other processes, from rainfall to which plants are able to grow where.
This scientific understanding of climate change has been the subject of incredible controversy. This is not because there is a lack of consensus among scientists. The controversy is essentially social: accepting that humans are causing rapid climate change means accepting the major predictions of serious environmental and social problems unless greenhouse gas emissions decline. For most people, accepting the scientific evidence makes not taking action indefensible.
Rapid climate change is having substantial effects on many processes now. As well as global air temperature increases, the ocean temperature has increased to depths in excess of 3,000 metres, mountain glaciers and snow cover have decreased across the world and the Greenland and Antarctic ice sheets are rapidly losing ice.
As a result of this, sea levels rose over the 20th century, and have been rising by 3mm per year over the past decade. There have been more intense and longer droughts, mostly in the tropics. Changes in extreme temperatures have been observed: hot days, hot nights and heat waves have become more frequent. People worldwide are being affected; for example, the 2003 heatwave in Europe killed 30,000 people.
Scientists have shown that human emissions of greenhouse gases doubled the probability of such a heatwave. Melting glaciers are increasing the size and number of glacial lakes, threatening mountain villages and towns as the lakes become increasingly likely to burst. Millions across the developing world are suffering due to droughts affecting crop yields. Plants and animals are being affected, as species move towards the Poles or up mountains in search of the cooler conditions they are adapted to live in. The oceans are becoming more acid as carbon dioxide dissolves into the water. Of 29,000 observational data sets of plants, animals and physical changes, 90% were consistent with the change expected by a response to global warming.
What about the future? Completely precise and accurate predictions are not possible as simulations of the response of the climate, ecosystems and oceans to natural changes and those caused by humans all contain a degree of uncertainty due to the complexity of the factors involved. However, the broad predictions are that under ‘business-as-usual', a globalised fossil-fuel intensive world, temperature rises of around 4 ° C are expected by 2100.
From this we can confidently expect reduced crop yields in the tropics, sea level rises and increases in flooding, more extreme weather events and at least a third of all species destined for extinction. More worryingly, the rapid rise in greenhouse gases in the atmosphere could set in motion large-scale and potentially abrupt changes in our planet's natural systems that may be irreversible.
The Amazon rainforest could die-off, the West Antarctic ice sheet could melt triggering massive sea-level rises. One key concern is that breaching one of these thresholds may increase the probability of then crossing another, which much of the global population would have extreme difficulty in adapting to.
Under business-as-usual are we heading for catastrophe? The most honest answer is ‘possibly'. It is currently impossible to make robust predictions about how future climatic changes will interact with social factors and other environmental problems in an increasingly globalised world.
But there are many socially explosive scenarios that are not far-fetched. For example, a future economic recession and geopolitical tensions over resources, coupled with extreme weather events causing a key crop failure and massive human migrations, could overload political institutions across many countries simultaneously. However, in terms of tackling climate change alone, it is physically possible to avoid the worst of climate change depending upon the political choices that are made now.
The need to reduce emissions of greenhouse gases is almost universally agreed. Even Rex Tillerson, the Chairman of Exxon Mobil, the world's largest oil company, says “...we should take steps now to reduce emissions...”. The key question is how much.
People increasingly want scientists to tell them what the ‘safe limit' is for carbon dioxide emissions to avoid serious problems. This is difficult because it is essentially a social and political question as what is ‘safe' is different for different people. Old people are more at risk from heatwaves. The poor cannot afford to move their homes if at risk of flooding, or repair them after flooding.
Essentially, choosing to reduce emissions implies balancing the risks of negative climate change against the risks of imposing measures that have negative impacts on people's lives. Many scientists say we need to limit temperature increases to a maximum of 2 ° C to avoid “dangerous interference with the climate system”, defined by the United Nations Framework on Climate Change as that which jeopardizes food security, protection of ecosystems and sustainable economic development. This is the target of the European Union and the British Government.
Recent research suggests that even greater reductions in greenhouse gas emissions than previously thought are necessary to achieve a 2 degree target because the capacity of the so-called carbon sinks – the overall absorption of carbon into oceans, vegetation and soils – is expected to reduce this century. To give a reasonable chance (more likely than not) of not exceeding 2 °, we need to stabilise carbon dioxide emissions at the most stringent level assessed by the Inter-Governmental Panel on Climate Change, around 400 parts per million.
Often this is expressed to include all different gases and aerosols known as ‘carbon dioxide equivalent' or CO2e. Using this measure the limit should be approximately 450 parts per million CO2e. Carbon dioxide concentrations alone are currently 380 ppm, rising by 2 ppm per year. To keep carbon dioxide concentrations from passing 400 parts per million requires global emissions to peak within the next 10 years, and globally to decline by between 50 and 80% by 2050. Allowing for equal per-person emissions for people across the world means a reduction in developed countries by about 90% by 2050 or sooner. This represents a reduction in carbon dioxide emissions of about 5 % per year, every year.
Dr Simon L. Lewis, Earth & Biosphere Institute, School of Geography, University of Leeds.
The author is a specialist on the interactions of tropical forests and climate change and a member of the Royal Society's Climate Change Advisory Group. All the scientific information included here appears in the IPCC Fourth Assesment Reports, available at www.ipcc.ch.