How Do We Know the Climate Is Warming? The Evidence Explained
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It is fair to ask how scientists can be so sure the planet is warming. Temperatures go up and down all the time, and measuring the "average temperature of Earth" sounds almost impossible. The confidence comes from many separate lines of evidence, gathered by different teams using different methods, that all point in the same direction.
Key points
- Several independent groups, including NASA, NOAA, the UK Met Office, Berkeley Earth and Copernicus, calculate global temperature. They agree closely.
- Global average surface temperature has risen by about 1.1 to 1.3 °C since the late 1800s, according to the IPCC and the WMO.
- The oceans have absorbed more than 90 percent of the extra heat, and ocean heat content keeps climbing.
- Shrinking glaciers, declining Arctic sea ice, rising sea levels and earlier springs all confirm the thermometer record.
Thermometer records
Reliable weather records go back to the mid-1800s in many places. Today, thousands of weather stations on land measure air temperature every day, while ships and floating buoys measure sea surface temperature across the oceans.
Turning these readings into a global average is not as simple as adding them up. Stations move, instruments change, and cities grow up around once-rural sites. Scientists handle this in a few ways:
- They use anomalies. Instead of averaging raw temperatures, they look at how much each station differs from its own long-term average. A mountain station and a beach station may have very different temperatures, but both can show the same change.
- They correct for known problems. When a station moves or changes equipment, a sudden jump appears in its record. Those jumps can be detected by comparing nearby stations and adjusted.
- They check for urban effects. Cities are warmer than the countryside, but studies that compare rural-only stations with the full record find very similar global trends.
Adjustments are sometimes portrayed as tampering. In fact, the largest single adjustment, to old ocean measurements taken with buckets, actually reduces the overall warming trend compared with the raw data.
Many teams, one answer
A key reason for confidence is that the global temperature record is not produced by one group. Several organizations build their own datasets, using different methods and partly different data.
| Dataset | Produced by | Approach |
|---|---|---|
| GISTEMP | NASA Goddard Institute for Space Studies | Station and ocean data, with gaps filled by interpolation |
| NOAAGlobalTemp | NOAA National Centers for Environmental Information | Station and ocean data with statistical reconstruction |
| HadCRUT | UK Met Office Hadley Centre and University of East Anglia | Station and ocean data with detailed uncertainty estimates |
| Berkeley Earth | Independent nonprofit research group | Started partly by scientists skeptical of earlier methods; uses a very large station set |
| ERA5 | Copernicus Climate Change Service (European Union) | A reanalysis that combines observations with a weather model |
These datasets differ slightly from year to year, by a few hundredths of a degree, but they show essentially the same long-term warming. The Met Office HadCRUT5 dataset, NASA's GISTEMP and NOAA's record all show the past decade as the warmest in their records. The World Meteorological Organization combines several of them in its annual State of the Global Climate reports.
Satellites
Since the late 1970s, satellites have measured the temperature of the lower atmosphere by detecting microwave radiation from oxygen molecules. These records are completely independent of ground thermometers. They show warming too, along with a cooling of the stratosphere higher up, a pattern expected from rising greenhouse gases. You can read why in What is the greenhouse effect?
Satellites also track sea level, ice sheet mass, sea ice extent and snow cover, giving a global view that would have been impossible a century ago.
Ocean heat: where most of the energy goes
Air temperature is only a small part of the picture. Water can store far more heat than air, and according to NOAA and the IPCC, the oceans have taken up more than 90 percent of the extra energy trapped by greenhouse gases since the 1970s.
Since the early 2000s, a global network of about 4,000 robotic floats called Argo has measured temperature down to 2,000 meters (about 6,500 feet). These measurements show ocean heat content rising steadily, with less year-to-year noise than surface temperatures. In many ways, ocean heat is the clearest single indicator of a warming planet.
Ice on land and sea
Ice responds to warming in ways anyone can see in photographs taken decades apart.
- Glaciers: Mountain glaciers are shrinking in nearly every region of the world, from the Alps to the Andes to the Himalayas.
- Ice sheets: Satellites that measure Earth's gravity show that both Greenland and Antarctica are losing ice overall.
- Arctic sea ice: According to the National Snow and Ice Data Center, the late-summer extent of Arctic sea ice has declined substantially since satellite records began in 1979, and the ice is also thinner and younger on average.
- Snow and permafrost: Spring snow cover in the Northern Hemisphere has decreased, and permafrost temperatures have risen in many monitored sites (see Permafrost thaw explained).
Rising seas
Sea level has risen by roughly 20 centimeters (about 8 inches) since 1900, and the rate has increased in recent decades. Two things drive this: water expands as it warms, and melting ice on land adds water to the ocean. Tide gauges and satellites both record the rise. For the full story, see Why sea levels are rising.
Nature's calendar
Plants and animals are independent witnesses. In many regions, spring flowers bloom earlier, birds migrate earlier, and the ranges of many species are shifting toward the poles or uphill. These changes are recorded by naturalists and scientists who were not studying climate at all, yet they match the temperature record.
What about uncertainty?
Every measurement has some uncertainty, and good science reports it. Early records are sparser, so uncertainty in 19th-century global temperature is larger than today. The exact amount of warming depends a little on the dataset and the baseline used, which is why numbers like "about 1.1 to 1.3 °C" are given as ranges.
What is not uncertain is the direction and rough size of the change. The IPCC's Sixth Assessment Report concludes that it is unequivocal that human influence has warmed the atmosphere, ocean and land.
The bottom line
No single measurement proves the climate is warming. The evidence is strong because many independent ones agree: thermometers on land and at sea, satellites, ocean floats, melting ice, rising seas and shifting seasons. Different teams using different methods reach the same conclusion. That kind of agreement is about as solid as science gets.