A new report out today says the world is on the brink of an irreversible climate disaster; quoting the United Nations Environment Programme, it says the global temperature will rise by 2.7°C by 2100.
According to NASA, the mean temperature on Earth is approximately 15°C or 59°F. But what does this all mean?
When we talk about Earth’s temperature, what we are really talking about is the average surface temperature of the entire globe.
Surface temperature measures how warm or cool the air is at the surface of the land or sea. From Singapore’s 30°C to Antarctica’s -30°C, temperatures across the world are collected every day and then combined to give a global average surface temperature, explains James Renwick, Professor of Physical Geography at Victoria University of Wellington, New Zealand.
“It’s actually quite hard to measure that … because absolute temperatures vary wildly with latitude, longitude, altitude,” the weather and climate researcher told Cosmos.
What’s really talked about in climate science is the change in average surface temperatures because the anomaly – the difference from the “normal” 30-year average temperature – varies a lot less, he explains.
Why are we interested in surface temperatures?
“Because it’s where we live,” says Renwick. “The surface temperature reflects the experience – the conditions that we’re experiencing … it’s a pretty good proxy for the whole state of the climate system.”
While temperatures are increasing down through the oceans and up through the atmosphere, it’s at Earth’s surface that we expect to see the biggest effects, says Renwick.
The global average temperature anomaly or the change in global average surface temperature is the key metric used to track how the climate is changing. It is also the metric used under the Paris Agreement – an international effort to combat climate change.
How are surface temperatures collected and calculated?
Temperature observations mostly come from weather stations, ships and buoys, and sometimes satellites. Observational data are then collated into datasets and analysed by different research institutes to calculate the global average temperature anomaly.
However, it’s not as simple as “adding it all up and dividing by the number of data points,” says Renwick, “we have to consider the area of the Earth’s surface that a particular data point accounts for.”
Where there are many observing stations, like in Europe, each observation would represent a small patch of land. By contrast, an observation from an island out in the middle of the Pacific, for example, would be assumed to account for a very large area.
Assumptions mean there are pitfalls. However, the global average temperature anomaly estimations from various research centres “all overlap very, very closely” despite using slightly different calculation methods, says Renwick.
Commonly cited datasets include NASA’s GISTEMP, NOAA’s MLOST and UK Met Office’s HadCRUT5.
What can we learn from tracking the global average temperature?
Since the pre-industrial era, the global average surface temperature has increased by about 1.2°C.
“Left to its own devices, Earth might warm up or cool down by a tenth of a degree from one year to the next, but a trend that’s pushing more than a degree away from the average 30 years ago is pretty remarkable.”
Renwick likens it to body temperature: “The body operates happily at 37°C. If you warm up by one degree, you’re probably not feeling too fresh. If you warm up by two degrees, you’re in trouble.
“And it’s like that with the Earth. A small change is a big deal.”
A main goal of the Paris Agreement is to limit global temperature increase to 1.5°C above pre-industrial levels.
According to the Intergovernmental Panel on Climate Change, limiting global warming to below 1.5°C will greatly reduce the risks and negative effects of climate change. Failing to do so will result in more frequent and severe extreme weather events including droughts, heatwaves, wildfires, heavy rains and floods.
