The planet’s climate is changing faster than ever before. This change is affecting everything we depend on, from natural habitats to human communities. It’s fueling environmental degradation, natural disasters and weather extremes, reducing food and water security, driving economic disruption and conflict, and pushing many people into poverty.
A warming climate is the result of human-caused emissions of greenhouse gases, which trap heat in Earth’s atmosphere and warm the surface. These gases occur naturally, but when we burn fossil fuels like coal, oil and gas – and clear forests for agriculture and development – we add large amounts to the atmospheric concentration of greenhouse gases.
During the past century, the average global temperature has risen about 1 degree Celsius. Climate change refers to the changes that are happening around the world, however – such as melting of ice sheets and sea levels rising – that go beyond just an increase in temperature. Climate change also includes changes in weather patterns such as droughts and floods, as well as shifts in ecosystems and the growing season.
Global temperatures are changing rapidly because we have altered the balance of energy that reaches Earth’s atmosphere from the Sun. Some of this energy is reflected back into space by bright surfaces such as clouds and ice, but most of it is absorbed by the atmosphere and re-emitted in long-wave infrared radiation that warms the air and ground. Incoming solar energy and outgoing radiative energy must be kept in balance for the Earth to remain livable.
Scientists have used a range of measurements and analyses, including the ratio of carbon isotopes, to determine that the increased concentration of carbon dioxide in Earth’s atmosphere cannot be explained by natural phenomena or variations in normal weather. Moreover, the elevated concentration of carbon dioxide in the atmosphere is unprecedented over the last 800,000 years.
The rate at which we will warm this century depends on future GHG emission trends, feedback processes that can dampen or reinforce disturbances to the climate system and unpredictable natural influences on the climate, like volcanic eruptions. Several scenarios for 21st century emissions – from stringent mitigation (RCP2.6) to high-emissions pathways (RCP8.5) – are projected in the CMIP5 model.
There are things we can do to reduce our impact on the climate, such as switching to renewable energy, using less water and turning off lights when not in use. But the biggest thing we can do is speak out: Call and write to your elected officials and tell them that climate change matters to you. They listen to their constituents, and the single most effective way to get them to take action is to make your voice heard.