Measuring the climate system from the ground, in the oceans and from space provides numerous signs of the Earth’s changing climate:
Atmospheric carbon dioxide concentrations:
Scientists have maintained a long-running observational record of the levels of carbon dioxide in the atmosphere since the late 1950s. Over this period atmospheric concentrations have increased from around 280 parts per million (ppm) to over 410 ppm today. Since the pre-industrial period atmospheric CO2 is estimated to have increased by nearly 50%. Evidence indicates that the combustion of fossil fuels and human alterations to the planet’s land surface explain all of this increase.
The present atmospheric CO2 concentration is expected to be the highest in at least 800 thousand years and is increasing at what is likely an unprecedented rate. The atmospheric concentrations of other heat-trapping gases have also increased alongside CO2. Concentrations of methane and nitrous oxide, the two largest contributors to anthropogenic forcing on the climate system after CO2, have increased by around 60% and 25% since their respective pre-industrial levels.
Global average surface temperature:
Climate change is most commonly measured using the average surface temperature of the planet. Measurements of near-surface air temperature from weather stations can be combined with measurements of ocean surface temperature from ships and buoys to create a record of the planet’s surface temperature going back to the mid-19th century.
Different estimates from the UK Met Office, NASA, NOAA and Berkeley Earth all show a rising trend in average global surface temperature over the last century despite using slightly different methodologies. Looking across datasets, the IPCC concluded that the average of the 2006-2015 decade was around 0.87°C (0.75°C to 0.99°C) above the average of the second half of the 19th century (an approximation for pre-industrial levels).
Year-on-year, natural fluctuations can be seen on top of this long-term warming. For this reason, scientists traditionally use a period of at least 30 years to identify a genuine climate trend.
Rising sea-levels and acidifying oceans:
More than 90% of the additional energy trapped in the climate system by raised greenhouse gas concentrations ends up in the oceans, contributing, along with melting ice on land, to rising global sea level. Recent observations indicate an increase in the rate of global sea-level rise since 1990. Sea levels around the UK are rising at a rate of around 1.4 mm per year.
The oceans have absorbed about 25% of the cumulative CO2 emissions into the atmosphere since the mid-19th century, increasing the acidity of the oceans. This has led to a decrease in ocean pH of around 0.1 since the pre-industrial period, around a 25% increase in acidity, which can affect the ability of coral organisms to build shells creating knock-on effects on whole marine ecosystems.
A changing cryosphere:
Satellite-based observations of Arctic sea-ice extent show a downward trend in all months of the year. September Arctic sea-ice extent (the month with the lowest ice extent of the year) has decreased by approximately 13% per decade since 1979. The ice in the Arctic has become both thinner and younger, with the fraction of Arctic sea-ice area that is more that 5-years old has decreased by 90% over the same period.
On land, the Greenland and Antarctic ice sheets have lost mass, contributing to observed global sea-level rise. Glaciers have shrunk, and areas of permafrost (ground that remains permanently frozen – which generally contain very large stores of carbon) have warmed to reach record high temperature.
Extreme weather events:
Across the globe land area as a whole there has been a measured overall decrease in the number of cold days and nights and overall increase in the number of warm days and nights. More areas with increases than decreases in the frequency, intensity and/or amount of heavy rainfall. Large parts of Europe, Asia and Australia and for the globe as a whole have seen detectable increases the in frequency or length of warm spells.