Climate change is a large-scale, long-term shift in the planet's weather patterns or average temperatures. Earth has had tropical climates and ice ages many times in its 4.5 billion years. So what's happening now?
Since the last ice age, which ended about 11,000 years ago, Earth's climate has been relatively stable at about 14 °C. However, in recent years, the average temperature has been increasing.
The information below details the seven main sources of evidence for climate change. You can find out more about the difference between weather and climate, what drives our climate and how our climate is changing in our climate infographic - What is climate change?
Scientific research shows that the climate - that is, the average temperature of the planet's surface - has risen by 0.89 °C from 1901 to 2012. Compared with climate change patterns throughout Earth's history, the rate of temperature rise since the Industrial Revolution is extremely high.
There have been observed changes in precipitation, but not all areas have data over long periods. Rainfall has increased in the mid-latitudes of the northern hemisphere since the beginning of the 20th century. There are also changes between seasons in different regions. For example, the UK's summer rainfall is decreasing on average, while winter rainfall is increasing. There is also evidence that heavy rainfall events have become more intensive, especially over North America.
Changes in nature
Changes in the seasons (such as the UK spring starting earlier, autumn starting later) are bringing changes in the behaviour of species, for example, butterflies appearing earlier in the year and birds shifting their migration patterns.
Sea level rises
Since 1900, sea levels have risen by about 10 cm around the UK and about 19 cm globally, on average. The rate of sea-level rise has increased in recent decades.
Glaciers all over the world - in the Alps, Rockies, Andes, Himalayas, Africa and Alaska - are melting and the rate of shrinkage has increased in recent decades.
Arctic sea-ice has been declining since the late 1970s, reducing by about 4%, or 0.6 million square kilometres (an area about the size of Madagascar) per decade. At the same time Antarctic sea-ice has increased, but at a slower rate of about 1.5% per decade.
The Greenland and Antarctic ice sheets, which between them store the majority of the world's fresh water, are both shrinking at an accelerating rate.
Why is our climate getting warmer?
Earth’s atmosphere has evolved to retain sufficient warmth from the sun to encourage a healthy, dynamic ecosystem, while shielding us from its harsher effects. The introduction of huge amounts of excess pollutants thickens this blanket of protective gases, causing heat to remain trapped within, rather than harmlessly escaping skywards. These gases can remain in our atmosphere for up to 90 years, contributing to long-term warming.
As the world warms, there are flow-on effects that can make things worse. For instance, warmer water melts polar ice caps each summer. Sea ice normally reflects heat from the sun, while water absorbs it. Less ice means more heat which in turn means less ice, leading to a cycle of warming from which it is hard to escape. Temperatures are already rising quickly, with the last decade being thehottest on record.
Are climate change and global warming the same thing?
Not exactly, but they’re closely related and some people use the terms interchangeably. Global warming causes climates to change. "Global warming" refers to rising global temperatures, while “climate change” includes other more specific kinds of changes, too. Warmer global temperatures in the atmosphere and oceans leads to climate changes affecting rainfall patterns, arctic ice (and the northern jet stream that drives our weather), frequency and intensity of storms and droughts, growing seasons, humidity, melting polar and glacial ice, and sea level.
Also, while “global warming” is planet-wide, “climate change” can refer to changes at the global, continental, regional and local levels. Even though a warming trend is global, different areas around the world will experience different specific changes in their climates, which will have unique impacts on local plants, animals and people. A few areas might even get cooler rather than warmer, and extreme weather events can include snowstorms as well as heat waves.
Is it too late to do something about climate change?
No, it’s not too late. In fact we must act to slow the progression of climate change, even if we can’t stop it completely due to greenhouse gases already in the atmosphere.
Our state and societies around the globe need to reduce greenhouse gas emissions to avoid worsening climate impacts and reduce the risk of creating changes beyond our ability to respond and adapt. Washington state is addressing this challenge and has adopted policies to reduce energy use, limit greenhouse gas emissions, and build a clean energy economy. (See Ecology website) Some changes in climate—and impacts on our state—are unavoidable, even if we reduce greenhouse gas emissions today.
By taking action now to respond and adapt to changing climate conditions, Washington can significantly limit the damage and reduce the long-term costs of the climate related impacts that are expected to grow in number and intensity in the decades to come.
What can I do about climate change?
There are many ways you can be aware and help slow climate change! From simple to more challenging actions you can take at home, when traveling, at work or school, and in your community as a climate-concerned citizen. See What YOU can do and make a commitment to do even more than you have already done.
Many people are innovating, contributing and taking action to slow climate change. Green energy is rapidly outpacing fossil fuels as the energy source of the 21st Century, and in 2010 for the first time global investments in green energy exceeded those in fossil fuels. In 2011, renewable sources supplied nearly 17 percent of global energy.
Why is our climate changing?
There are many factors that could cause a change in our climate.
Anything that affects the amount of energy being absorbed from the Sun, or the amount being radiated by the Earth - the planet's energy balance - may produce long- or short-term cooling or warming.
Natural or 'forcing'
An imbalance in the planet's 'energy account' can be caused by changes in the energy radiated by the Sun, changes in greenhouse gases, particles or clouds, or changes in the reflectivity of the Earth's surface. Imbalances caused by these changes are often called 'forcings'. A positive climate forcing will tend to cause a warming, and a negative forcing a cooling.
Changes in climate can also arise from variations within the climate system. For example, the El Niño/La Niña system, in which interactions between the oceans and the atmosphere cause global temperature changes lasting a number of years. This is natural variability.
Relatively small changes in the Earth's energy account can lead to further changes, and these can further modify, for example, the reflectivity of the Earth or the amount of water vapour. The climate system is therefore highly sensitive to small changes, as these often 'feedback', and have large, long-term effects on the climate.
Increased solar energy
Scientific research into the energy we receive from the Sun has found that it is not the main cause of the current warming trend. However, solar radiation is thought to have been responsible for increased warming early in the 20th century.
Greenhouse gas increase
There's overwhelming and growing evidence that the warming is due to vastly increased - and still increasing - quantities of greenhouse gas in the atmosphere.
The most important greenhouse gas, in that it has the strongest greenhouse effect, is water vapour. It increases in concentration as the atmosphere warms. The amount of water vapour in the atmosphere has increased, but there's no reason for this scale of change other than the increase in temperature.
Carbon dioxide (CO2) and methane are both important greenhouse gases, which have a 'forcing' effect (they increase the effect of warming). Their increase in concentration is mainly caused by emissions from human activity. However, there are also potentially large secondary effects, for example decreased carbon storage due to reduced forest growth or the potential release of large amounts of methane from permafrost, caused by raised temperatures.
The amount of CO2in the atmosphere has increased dramatically - by about 38% (as of 2012) - since the Industrial Revolution. As we continue burning fossil fuels and other activities, the amount of CO2will continue to rise. This means the extra CO2will absorb and emit more and more of the Earth's outgoing radiation, and this will further warm our climate. As the atmosphere warms, the amount of water vapour it holds also increases - which further adds to the warming effect.
Methane has a strong greenhouse effect, but it doesn't stay in the atmosphere for more than about a decade. CO2lasts for about 100 years or more, meaning it has a very long time to build up and affect our climate. Some of the CO2in our atmosphere was emitted before World War I.
Cutting down forests, one of the major natural storage 'sinks' for carbon, is further increasing the imbalance between the CO2we emit and the planet's capacity to re-absorb it.
Why would we care?
Here are a few simple examples of climate changes due to global warming:
As temperatures rise, more moisture evaporates from the oceans (like water evaporates faster as heat is added to a kettle). The air above the oceans now holds 4 percent more water vapor than 30 years ago. Bigger storms are one result.
Warmer air over land evaporates more water from soil and plants and can create or extend drought.
Global warming has increased the loss of summer sea ice in the Arctic, which has altered atmospheric conditions that influence harsher winter weather in the U.S. and Europe.
Warmer air also warms glacier ice in Greenland and other lands and that melting water adds to sea level rise, as does the expansion of warmer ocean water.
Warmer air melts permafrost (previously permanently frozen ground in northern latitudes) and melted permafrost releases methane from ancient decayed vegetation in thawing soils. Methane is another potent greenhouse gas which further adds to global warming.
One of the biggest ways people contribute to greenhouse gases is by burning fossil fuels. We use coal, oil, and natural gas to generate electricity, heat our homes, power our factories, and run our cars. Changing land use patterns contribute, too. Trees and other plants absorb carbon dioxide and give off oxygen. When trees are cut down for development, agriculture, and other purposes, they’re no longer available to take carbon dioxide out of the air, and actually release carbon dioxide as they decay or burn.
Intergovernmental Panel on Climate Change – Climate Change 2007: The Physical Science Basis, Summary for Policy Makers; 2007 Climate into the 21st Century, World Meteorological Organization, 2003, University Press, Cambridge UK, pg 203 (Climate into the 21st Century, World Meteorological Organization") Scientific American, December 2012, Linking Climate and Weather, Charles Greene