Why should we care about one degree of warming? After all, the temperature fluctuates by many degrees every day where we live. The temperatures we experience locally, and in short periods, can fluctuate a lot due to predictable cyclical events like night and day, summer and winter, and hard-to-predict wind and precipitation patterns.

Unlike these local, short-term changes in climate, the global temperature record represents an average over the entire surface of the planet. And unlike short-term fluctuations, the global temperature mainly depends on how much energy the planet receives from the Sun and how much it radiates back into space - quantities that change very little. The amount of energy radiated by the Earth depends significantly on the chemical composition of the atmosphere, particularly the amount of heat-trapping greenhouse gases.

A one-degree global change is significant because it takes a vast amount of heat to warm all the oceans, atmosphere, and land by that much. In the past, a 1°C to 2°C decrease was all it took to plunge the Earth into the Little Ice Age. And, a 5°C drop was enough to bury a large part of North America under a towering mass of ice 20,000 years ago.

With the current level of GHG emissions, scientists expect each successive decade to be warmer than the previous. Global temperatures have risen from 1950 through the end of 2013, and it has been 38 years since the recording of a year of cooler than average temperatures. The average temperature in 2013 was 14.6°C (58.3°F), which is 0.6°C (1.1°F) warmer than the mid-20th century baseline. The average global temperature has risen about 0.8°C (1.4°F) since 1880.i

iSteve Cole and Leslie McCarthy. 2014. "Long-term global warming trend sustained in 2013." NASA Global Climate Change, Features January 21.