Global mean temperature is an average of temperature across the globe. It is usually expressed as a temperature “anomaly” which is just the difference from the average over a fixed period. Temperature is measured at weather stations, by ships and buoys and by satellites.
Temperature is a key climate indicator. The average temperature of a region and how it changes through the year have an important determining effect on how people live, the kinds of crops they can grow and the design of buildings and other local infrastructure. Changes from year to year can affect food production and the spread of pests and disease. Extremes of heat and cold can adversely affect people's health and food security as well as infrastructure such as roads, rail tracks, and energy demand.
Ocean water temperature has a similarly important role to play for marine life. Recent high water temperatures led to the bleaching of large areas of the Great Barrier reef. Ocean temperatures shape the formation and development of hurricanes which typically form only over warmer waters. Temperature patterns in the ocean, such as El Niño, change weather in many locations around the world, some far removed from the Pacific Ocean where it originates.
Temperature measurements made over land and in water across the globe tell us how temperature has changed over years, decades and even centuries. They also tell us how temperatures vary from one place to another. Global temperature is an aggregate of all the local temperature changes. It is a summary of the overall tendency in surface temperature and it is probably the longest instrumental series of global climate change that we have.
Global mean temperature is a key indicator in the context of the Paris Agreement. The central aim of the Paris Agreement "is to strengthen the global response to the threat of climate change by keeping a global temperature rise this century well below 2 degrees Celsius above pre-industrial levels and to pursue efforts to limit the temperature increase even further to 1.5 degrees Celsius."
Note that the Paris Agreement limits refer to long-term averages and not to individual years. A single year exceeding 1.5°C or 2°C would not indicate that that limit had been passed. Likewise, the Agreement references global temperature and not local temperatures.
Overall, average temperatures have increased in most areas since the start of the 20th Century. This is reflected in the long-term increase in global temperature. However, temperatures have not increased at the same rate everywhere. Some general patterns of warming are to be noted: the land has generally warmed faster than the oceans; and over the past two decades, the Arctic has warmed twice as fast as the rest of the world.
Temperatures have not increased steadily. There have been periods when global temperatures warmed faster and other periods when they warmed more slowly or even cooled. For instance, from the 1940s to the mid-1970s, global temperatures were relatively stable. From the late 1970s to the present day, global temperatures increased at a higher rate, punctuated by slight dips associated with large volcanic eruptions and possibly with shifts in the Pacific Decadal Oscillation.
Climate modes such as El Niño and La Niña can modify temperatures around the world. El Niño tends to boost global temperature temporarily and La Niña to suppress it. Large volcanic eruptions which inject volcanic ash and other compounds into the upper atmosphere can lead to a cooling of the global climate for a few years after the eruption.
Extremes of temperature have evolved too. The changes are largely in the direction we would expect them to be given the overall increase in temperature. Averaged across the globe, we have witnessed more warm days and nights and fewer cold days and nights.
Air temperature is measured at weather stations around the world. It is usually measured by a thermometer in a protective screen. The screens have slotted sides which allow air to circulate freely but prevent the sun from shining directly on the sensor. This means that the measured air temperature can be different to the temperature a human observer “feels”, which also depends on the humidity, wind and sunshine.
It is generally recommended that weather stations are situated in areas where the environment around them does not change as this could affect the temperatures measured at the station. Despite the best efforts of meteorological organisations around the world, this is not always possible and sometimes stations have to be moved. Consequently, it is necessary to process the data to estimate how temperatures would have changed if the environment around the station had stayed constant.
Historically, thermometers were housed in screens of different designs. During the 19th Century, thermometers in some areas were kept beneath thatched shelters. The instruments themselves have changed too, with mercury-in-glass thermometers becoming rarer while electronic instruments have become more common. These kinds of changes can affect the recorded temperatures.
Sea-surface temperature – used instead of air temperature over the oceans – is measured by ships and buoys. Vessels in the Voluntary Observing Ship Programme make measurements of sea-surface temperature on their travels as well as a range of other marine meteorological measurements. Drifting and moored buoys also make sea-surface temperature measurements and are often put in (or drift into) places that are seldom visited by ships, giving us a more global picture.
In combining these measurements to produce global temperature data sets, it is necessary to account for the uneven distribution of measurements, which are disproportionately clustered in certain areas. The teams that produce global temperatures have a number of different methods for dealing with this issue. Despite the diverse means deployed to estimate global temperature, the series produced by the different institutes are in very good agreement with each other, suggesting that the estimated temperature change is reasonably robust.
The Intergovernmental Panel on Climate Change (IPCC) concluded that "most of the observed increase in global average temperatures since the mid-20th century is very likely due to the observed increase in anthropogenic greenhouse gas concentrations." There is more uncertainty about the causes of the changes observed in the early part of the 20th-century. Possible influences include solar, volcanic and greenhouse gas factors, and natural variability that is internal to the climate system.
Year-to-year variations in global temperature around the slower, long-term changes are chiefly associated with El Niño events (which temporarily increase global temperature) and La Niña events (which temporarily reduce it). Short excursions lasting a month or two are common during northern hemisphere winter, where changes in the prevailing winds can lead to rapid and strong but short-lived warming or cooling over Asia and North America.