Over more than a century, global average temperature warmed 1.5°F (0.8°C).

For more than 100 years, air temperature has been measured by weather observers all over the world. Averaging these daily measurements over a whole year let us see long-term trends instead of short-term weather events.  And the long-term trend is that Earth is warming over time. Data collected from weather balloons (since the 1930s) and satellites (since the 1970s) also shows warming of Earth's climate.

Temperature is warming because the amount of heat-trapping gases in the air is increasing.

In 1958, Charles Keeling started taking daily measurements of the amount of carbon dioxide, a heat-trapping greenhouse gas.  He wanted to study the seasonal cycle that drives carbon dioxide higher when plants are dormant during Northern Hemisphere winter and lower in the summer when plants are taking carbon dioxide out of the air through photosynthesis. But Keeling found an unexpected change over time – that the total amount of carbon dioxide was increasing year after year.

The amount of heat-trapping gases has increased as fossil fuels are burned and concrete is made.

Driving cars that burn gasoline and making electricity by burning coal and gas releases carbon dioxide and other greenhouse gases into the atmosphere. Curing of cement emits carbon dioxide, too.  Since the late 19th Century, when factories powered by coal became common, the amount of carbon dioxide released into the atmosphere each year has increased. The graph above shows how emissions have increased over time. Note that carbon dioxide can stay in the atmosphere for a long time, so emissions year after year add up. To get the amount of carbon dioxide in the atmosphere to stop increasing will take a decline in emissions similar to the increase since the 1950s. 

There are natural factors that can change Earth’s temperature like sunspots and volcanic eruptions, but these factors aren’t responsible for current warming. In fact, it would be a little bit cooler than it is today if the Sun and volcanoes were controlling the climate.

If we keep emitting greenhouse gases, climate will keep warming.

How much climate change we experience depends on how quickly we act during this century. If we continue to emit greenhouse gases, we'll see more far more warming during this century than we saw over the past century according to projections from Earth system models. Global average temperature will increase between 3 and 5° Celsius (5.4 to 9° F) . The good news is that this doesn't have to happen. If we find ways to spew far fewer heat trapping gasses into the air, global average temperature will increase less than 1° Celsius this century, which will keep Earth a livable place. 


Next, read about how warming is affecting other aspects of our climate like precipitation, as well as the ocean, ice, ecosystems, and health.