Thousands of weather stations worldwide contain thermometers that keep track of surface air temperature day and night. Scientists sift through these measurements to ensure their accuracy, checking for errors. For example, a station recording 45 °C in London would need to be investigated. Scientists combine the individual measurements into a global average. The process of constructing the Earth’s global temperature record has been carried out several times by different groups of scientists. All the main calculations agree on the long-term warming trend: the global average surface air temperature has increased by about 0.7 °C since 1900.

Ships began measuring ocean temperatures in the late 1800s. Since the 1990s, scientists have also been launching floating thermometers called Argo floats. By 2007 three thousand of these instruments were bobbing around the world’s oceans, tracked by satellites. Measurements from ships and Argo floats show ocean temperatures increasing. The rise in ocean temperature is slower than the rise in surface air temperature. This is partly because water has a high heat capacity and so warms more slowly than air, and partly because it takes time for the heat to travel through the ocean’s vast depth. Even so, global sea surface temperatures have increased about 0.6 °C since 1900.

Global warming doesn’t mean temperatures rise at the same rate across the globe. For example, ocean temperatures have risen less than air temperatures, because water warms up more slowly and the ocean has much more mass than the atmosphere. And central Antarctica has seen hardly any warming, mainly because it’s isolated from the rest of the world by a circle of strong winds. At the other extreme, Arctic temperatures have risen nearly twice as much as the global average. This is mostly due to a feedback effect: melting white sea ice uncovers darker water which absorbs more sunlight, accelerating the warming.

From about 1940 to 1970, there was a pause in the long-term global warming trend. Why? Scientists think the warming in the early 1900s was partly caused by an increase in the Sun’s output and a decrease in volcanic eruptions. Human activities also contributed, but carbon dioxide (CO₂) emissions were much smaller at that time. Mid-century, the Sun’s output stabilised and volcanic activity picked up again. Humans were also emitting large quantities of aerosols in pollution and smog, cooling the planet through global dimming. Ocean cycles may have caused some cooling too. These effects temporarily cancelled out the warming effect of increasing CO₂. But as humans emitted more and more CO₂, the warming became dominant. Since 1970 global surface air temperature has increased significantly – by about 0.5 °C.

The decade from 2000 to 2010 was the warmest on record, but there was no significant rise in temperature over those 10 years. Has global warming stopped? No: 10 years is a short time in climate terms. To calculate a meaningful trend in global temperature, scientists use measurements spanning 20 years or more. The difference between 10 and 20 years doesn’t sound like much, but statistically it can be crucial. Over a few years, the climate can fluctuate because of short-term natural cycles such as El Niño. This means trends over a single decade often differ from the long-term trend. The global temperature trend from 1990 to 2010 shows significant warming.

The ‘urban heat island effect’ is the higher urban temperatures caused by heat from buildings being concentrated in one place. This could make a weather station in a growing city show a rise in temperature, giving the impression that the whole region is warming up, even if it isn’t. Scientists take great care to remove these effects from global temperature measurements. Several scientific studies have compared urban and rural temperature trends, finding only tiny differences between them. Where there are differences scientists usually adjust the urban trends downwards to match trends recorded by nearby rural weather stations. Many rigorous analyses have confirmed that global warming is not just a by-product of urbanisation.

Tamsin Grey is measuring the impacts of climate change in the Antarctic Peninsula – one of the fastest-warming areas of the world. Every day she is constantly taking measurements of the local weather conditions. ‘Sometimes I use a ski plane to set up weather stations in places where no-one has ever been before,’ she says.

It can be tough: ‘Sometimes I have to spend up to a year in Antarctica without seeing my family. But it’s all worth it. My research has helped scientists find out that this part of the world is warming about ten times faster than the global average.’

• NASA: Changes in Land Surface Temperature
• Met Office: Global Temperature  Trends
• NOAA: Argo Floats