'Climate is an angry beast...' Quote by climate scientist Wallace Broecker
Image by Lisa Brewster

Everyone’s heard that the planet’s climate is changing but is it true that the planet is warming? What’s the evidence? If there’s an unusually cold winter, isn’t that evidence of global cooling? Many people are sceptical and a little confused. Is global warming just another scare story put about by green eco-nutters? It’s more comforting to believe that everything’s fine and we can carry on as usual. But an unpalatable truth is that the global economic system depends almost totally on cheap fossil fuels – coal, oil, natural gas – to power industry, transport, modern consumerist lifestyles and provide employment. Taking action to reduce the greenhouse gases (GHGs) - which science says cause climate change - will mean drastically cutting back on using these fuels. There’s trouble ahead. So it is reasonable to question how we know climate change really is happening. Mitigation and adaptation will dramatically change our lifestyles, though not necessarily for the worse. So what really is the evidence for climate change? This 4-minute guide summarises it.

Climate and the weather: There is now a mass of evidence that climate is changing fast. Confusion arises because most people don’t appreciate the difference between weather and climate. A cold winter in north Europe doesn’t mean that the climate is cooling: there’s a lot of natural variation year by year and always has been. Climate is about averaging the weather’s variations around the planet over a number of years and looking for a global trend. And there is a trend: temperatures are increasing. The planet is getting hotter and the rate looks set to accelerate.

The evidence comes from careful observations by scientists from many different disciplines over many years. Many lines of evidence can actually be seen happening:

  • Ice sheets and glaciers are melting everywhere and there are many dramatic before and after photos which illustrate this
  • The area covered by floating sea ice in the Arctic is reducing rapidly
  • Permafrost in the Arctic is melting, releasing methane, a potent greenhouse gas (an example of a dangerous ‘positive’ feedback)
  • The lower atmosphere (troposphere) is becoming warmer
  • Sea levels and ocean temperatures are rising (see below)
  • Species of animals and plants are ‘migrating’ to higher latitudes because their home ranges are becoming too warm for them. Diseases are also expanding their range and affecting crops and trees as well as people
  • Coral reefs are being killed by the hotter waters. Corals are not only beautiful to look at, they are nursery grounds to myriads of marine species (and sometimes called ‘the rainforests of the sea’.) The planet needs its corals because they sequester carbon from carbon dioxide (CO2) to build their skeletons out of a hard, white mineral called calcium carbonate so, like trees, they are ‘carbon sinks’
  • The oceans are absorbing much of the CO2 but as they do so, they are becoming more acidic. This is affecting all kinds of marine life which build their shells out of calcium carbonate. The mineral dissolves in weak acid so acidification means that corals and shells won’t be able to grow, triggering all kinds of knock-on effects in the marine food chain.

Predicting the future: Global Climate Models Climate scientists have developed computer models to predict future climate. They know these are generally accurate because they can successfully be used to predict known past climate by checking their predictions against actual observations (see below). The models allow scientists to predict how the climate will change over the next few decades and are a cornerstone of periodic updates from the Intergovernmental Panel on Climate Change (IPCC) on global climate change.

How can scientists investigate past climates accurately? One way is to examine drill cores taken from ice sheets like those covering Antarctica and Greenland. Past climates can be reconstructed effectively using the records of former atmosphere composition and precipitation preserved in the ice. What’s more, they can be cross-checked using actual historical records and other ‘proxy’ observations such as tree-rings, isotope analysis and radiometric dating. Importantly, the ice cores contain a record of CO2 levels which are higher now than at any time in the last 700,000 years. One well-known result of using all these different methods to assess past climates is the hockey stick graph in which numerous different lines of evidence broadly agree that temperatures have over recent decades started on a steep upward trend. It is not a uniform upward movement because of complex atmosphere-ocean oscillations, the best-known of which is El Niño.

One prediction made by the computer models is that the Arctic and Antarctic will warm faster than the rest of the world. Evidence is coming in that not only is this happening but, alarmingly, it’s happening even faster than predicted because of positive feedbacks. Other predictions show droughts and desert areas increasing (particularly in Australia) and more violent weather patterns with poor countries particularly vulnerable (especially much of Africa). Tropical forests - normally massive carbon 'sinks' (the trees absorb CO2 from the air and transform it into wood, so locking up the carbon) – are today being logged and burned to make way for farming and biofuel plantations, releasing vast quantities of CO2 into the air. As if that wasn’t enough, the models predict drying and major die-off of the Amazon rainforests and increase in wildfires in these former sanctuaries of biodiversity.

The main concern is that rising global temperatures will trigger ‘tipping points’ where GHG inputs reach a critical level, causing a major climate ‘flip’ which could be extremely hostile to much of life – including humans. We know from the distant past that major climate change events can and do occur. One of these, almost certainly caused by GHGs from stupendous volcanic eruptions, wiped out 90 per cent of life on the planet. This mass extinction event occurred around 250 million years ago and was probably worsened by ‘tipping points’ such as major methane releases from methane clathrates. (Today’s oceans host vast deposits of clathrates.) We know of 5 mass extinctions from the geological record and we are now causing the sixth.

How warming happens: the greenhouse effect If you enter a greenhouse on a sunny day, it’s hot because the sun’s heat is trapped by the glass. Carbon dioxide (and other gases like methane, nitrous oxide and ozone-killer CFCs) are called greenhouse gases because they, like the glass in a greenhouse, trap some of the sun’s heat. Without the greenhouse ‘blanket’, the planet would radiate most of this heat back into space. As more GHGs gush into the atmosphere from power station chimneys, farming and car tailpipes, it’s rather like adding double glazing to the greenhouse: more heat is trapped. Most of this heat is absorbed by the world’s oceans so they, like the air, are getting hotter.

The bathtub effect: Without the greenhouse effect, life on Earth wouldn’t exist. Some GHG are essential to keep the planet habitable, but humans are grossly overdoing it. Imagine a bath (which represents the atmosphere) with the taps full on and gushing water (representing GHGs pouring into the atmosphere). There’s no plug so water is also draining from the plughole (representing carbon ‘sinks’ like the oceans and forests which both naturally absorb CO2). In a stable system, the amount of water coming in is roughly balanced by the amount flowing out: the carbon cycle. But we’ve upset the system by pouring increasing amounts of ‘water’ into the ‘bathtub’ so the tub is filling up and will soon overflow. The ‘carbon sinks’ drain is overwhelmed so the planet heats up. This is well explained by the Bathtub simulator. Before people began to burn fossil-fuel in the 19th century, CO2 levels – even during warm periods - were below 300 parts per million (ppm). During ice ages, they fell to less than 200ppm. Since the industrial revolution, they have risen ever faster, particularly in the last decade and now stand at 387. Actual warming closely mirrors this rise.

Sea level rise: Warmer water expands so sea levels go up. But sea levels also rise because of all the melting glaciers and ice sheets around the world. In fact, the rapid melting of almost all the world’s glaciers is one of the most scary indicators that the climate is warming. Sea levels have been rising by about 2mm each year for the last century but this is predicted to greatly increase, causing large scale flooding of many low lying populated areas. The IPCC in their latest (2007) report predict about half a metre of further sea level rise though more recent research suggests double that amount.

This guide to the scientific evidence for climate change and the predictions science can make is deliberately very brief. It first appeared on OneClimate.net. Below is a list of sources of further information if you want to follow anything up.

The Royal Society has produced this overview of the current state of scientific understanding of climate change to help non-experts better understand some of the debates in this complex area of science.

New Scientist's guide to climate change, global warming and greenhouse gases with many other interesting links and news stories.

‘Understanding and Responding to Climate Change’ Downloadable PDF document from the US National Academies. Excellent guide with clear explanations and many images. A free printed version is also available.

RealClimate Climate science blog written by climate scientists with many useful short guides e.g. ‘Highlight’ (right column, scroll down)

Climate change for kids, explained by OneWorld’s Tiki the Penguin

OneWorld’s guide to climate change exposes the reality that global warming will impact poorer countries harder and sooner than the richer countries which are responsible.