Global warming is the current rise in temperature of the air and oceans. It happens because humans burn coal, oil and natural gas, and cut down forests. Average temperatures today are about  higher than before people started burning a lot of coal around 1750. In some parts of the world it is less and some more. Most climate scientists say that by the year 2100 temperatures will be  to   higher than they were before 1750. The extra warmth melts ice caps around the world. Sea level is rising for two reasons: ice on the land, like Greenland, melts into the sea. Water also gets larger when it warms. Many cities will be partly flooded by the ocean in the 21st century. 

Global warming is mostly because of people burning things, like gasoline for cars and natural gas to keep houses warm. But the heat from the burning itself only makes the world a tiny bit warmer: it is the carbon dioxide from the burning which is the biggest part of the problem. Among the greenhouse gases, the increase of carbon dioxide in the atmosphere is the main cause of global warming, as predicted by Svante Arrhenius a hundred years ago, confirming the work of Joseph Fourier more than 200 years ago. When people burn fossil fuels like coal, oil and natural gas this adds carbon dioxide into the air. This is because fossil fuels contain lots of carbon and burning means joining most of the atoms in the fuel with oxygen. When people cut down many trees (deforestation), this means less carbon dioxide is taken out of the atmosphere by those plants.

As the Earth's surface temperature becomes hotter the sea level rises. This is partly because water over  expands when it gets warmer. It is also partly because warm temperatures make glaciers and ice caps melt. The sea level rise causes coastal areas to flood. Weather patterns, including where and how much rain or snow there is, are changing. Deserts will probably increase in size. Colder areas will warm up faster than warm areas. Strong storms may become more likely and farming may not make as much food. These effects will not be the same everywhere. The changes from one area to another are not well known.

Governments have agreed to keep temperature rise below , but current plans by governments are not enough to limit global warming that much.

People in government and the Intergovernmental Panel on Climate Change (IPCC) are talking about global warming. But governments, companies, and other people do not agree on what to do about it. Some things that could reduce warming are to burn less fossil fuels, grow more trees, eat less meat, and put some carbon dioxide back in the ground. People could adapt to some temperature change. The Kyoto Protocol and Paris Agreement try to reduce pollution from the burning of fossil fuels. Most governments have agreed to them but some people in government think nothing should change. The gas produced by cows digestion also causes global warming, because it contains a greenhouse gas called methane.

Temperature changes 

Climate change has happened constantly over the history of the Earth, including the coming and going of ice ages. But modern climate change is different because people are putting carbon dioxide into the atmosphere very quickly.

Since the 1800s, people have recorded the daily temperature. By about 1850, there were enough places measuring temperature so that scientists could know the global average temperature. Compared with before people started burning a lot of coal for industry, the temperature has risen by about . In 1979, satellites started measuring the temperature of the Earth.

Before 1850, there were not enough temperature measurements for us to know how warm or cold it was. Climatologists use proxy measurements to try to figure out past temperatures before there were thermometers. This means measuring things that change when it gets colder or warmer. One way is to cut into a tree and measure how far apart the growth rings are. Trees that live a long time can give us an idea of how temperature and rain changed while they were alive.

For most of the past 2000 years the average temperature of the world didn't change much. There were some times where the temperatures were a little warmer or cooler in some places. One of the most famous warm times was the Medieval Warm Period and one of the most famous cool times was the Little Ice Age (not really an ice age). Other proxy measurements like the temperature measured in deep holes mostly agree with the tree rings. Tree rings and bore holes can only help scientists work out the temperature back to about 2000 years ago. Ice cores are also used to find out the temperature back to about half a million years ago.

Greenhouse gases 

There a many greenhouse gases that cause the Earth to warm. The most important one is carbon dioxide (CO2). CO2 comes from power plants, where coal and natural gas are burnt. Cars also emit CO2 when they burn petrol. About 23 billion tons of carbon dioxide and other greenhouse gases are released into the Earth's atmosphere each year. The amount of CO2 in the air is about 48% more than it was around 1750. About three-quarters of the CO2 that people have put in the air during the past 20 years are due to burning fossil fuel like coal or oil. The rest mostly comes from changes in how land is used, like cutting down trees. 

The second most important greenhouse gas is methane. This comes mostly from food production. Some methane also leaks when oil and gas are produced.

Dust and dirt 
Dust and dirt in the air may come from natural sources such as volcanos, erosion and meteoric dust. Some of this dirt falls out within a few hours. Some is aerosol, so small that it could stay in the air for years. The aerosol particles in the atmosphere make the earth colder. The effect of dust therefore cancels out some of the effects of greenhouse gases. Even though humans also put aerosols in the air when they burn coal or oil this only cancels out the greenhouse effect of the fuel burning for less than 20 years: the carbon dioxide stays in the atmosphere much longer and keeps on warming the earth.

Slowing climate change 
Some people burn less fossil fuel. Countries try to emit less greenhouse gases. The Kyoto Protocol was signed in 1997. It was meant to reduce the amount of greenhouse gases in the atmosphere to below their levels in 1990. However, carbon dioxide levels have continued to rise.

Energy conservation is used to burn less fossil fuel. People can also use energy sources that don't burn fossil fuel, like solar panels or electricity from nuclear power or wind power. Or they can prevent the carbon dioxide from getting out into the atmosphere, which is called carbon capture and storage (CCS). Geoengineering is also seen by some as one a way to slow or stop climate change.

Dealing with impacts of climate change 
People can change how they live because of the impacts of climate change. For example, they can go to places where the weather is better, or build walls around cities to keep flood water out. This cost money, and rich people and rich countries will be able to change more easily than the poor.

History of climate change science 
As early as the 1820s a lot of scientists were finding out about climate change. Joseph Fourier believed that light from the sun can enter the atmosphere, but cannot leave nearly as easily. He tried to prove that air can absorb the infrared radiation and will be given back to the Earth’s surface. Later in 1859, John Tyndall discovered that water vapor and CO2 trap heat waves given by the sun. In 1896, Svante Arrhenius tried to prove that it would take thousands of years for the industrial production of CO2 to raise the Earth’s temperature 5-6°C.  But throughout the early 20th century many people did not believe this idea because it was too simple. In the mid 20th century, scientists worked out that there was a 10% increase in carbon dioxide in the atmosphere over the 19th century, which made it a little bit warmer. It was at this time that people believed the emissions of CO2 would increase exponentially in the future and the oceans would absorb any surplus of greenhouse gases. In 1956, Gilbert N. Plass decided that greenhouse gas emissions will have an effect on the Earth’s temperature and argued that not thinking about GHG emissions would be a mistake. Soon after, scientists studying all different kinds of science began to work together to figure out the mystery of GHG emissions and their effects. As technology advanced, it was in the 1980s that there was proof of a rise in CO2 levels. An ice core, captured through drilling, provided clear evidence that carbon dioxide levels have risen.

Effects of global warming on sea levels 
Sea level is rising because water over  gets bigger when it gets warmer. Also because the Antarctica and Greenland ice sheets are melting. Sea level will rise between half and one metre by 2100, and between 2 and 7 metres by 2300.

Low-lying areas such as Bangladesh, Florida, the Netherlands and other areas face massive flooding.

Cities affected by current sea level rise 
Many cities are sea ports and under threat of flooding if the present sea level rises.

These and the other cities have either started trying to deal with rising sea level and related storm surge, or are discussing this, according to reliable sources.
 London 
 New York City
 Norfolk, Virginia, in Hampton Roads area of United States 
 Southampton 
 Crisfield, Maryland, United States 
 Charleston, South Carolina 
 Miami, Florida, has been listed as "the number-one most vulnerable city worldwide" in terms of potential damage to property from storm-related flooding and sea-level rise.
 Saint Petersburg 
 Sydney, Australia 
 Jakarta 
 Thatta and Badin, in Sindh, Pakistan 
 Malé, Maldives
Mumbai, Buenos Aires, Los Angeles, Rio de Janeiro 

Also, all other coastal cities are in danger.

Further reading
 What is climate change  National Geographic Kids

 What is climate change? A really simple guide BBC

Related pages 
 Climate change
 James Hansen
 Stern Review
 Pollution
 Intergovernmental Panel on Climate Change
 Greenhouse effect

References

Other websites 

 The Climate Change Guide easy-to-understand information on Climate Change

Energy
Climate change
Air pollution