The Earth’s climate system comprises interacting physical components — the atmosphere, the hydrosphere (liquid water on Earth), the cryosphere (frozen elements), the land surface, and the biosphere, which encompasses all living things on land and in water. Measurements of variables within all of these systems provide independent lines of evidence that the global climate system is warming. The consistency of the signals across multiple components of the climate system provides a compelling story of unequivocal change.

The best-known indicator for tracking climate change is global mean surface temperature (GMST), estimated as the average (or mean) temperature for the world from measurements of sea surface temperatures and of near-surface air temperatures above the land. This measure has risen an estimated 0.85°C (90% uncertainty range between 0.65°C and 1.06°C) over the period 1880–2012. Each of the last three full decades (1980s, 1990s, and 2000s) has broken successive records for average 10-year temperatures. A warming slowdown occurred in the early 21st century, even though decadal temperature for the 2000s was higher than that for the 1990s. Natural climate variability influences GMST on a variety of timescales; therefore, periods of reduced or enhanced warming on decadal timescales are expected. The causes of the early 21st century warming slowdown are now better understood, and it appears to have ended, with the years 2015, 2016, and 2017 being the warmest on record, with GMST more than 1°C above the pre-industrial average level.

Signals of climate warming are also evident in other components of the climate system. The shift toward a warmer global climate on average has been accompanied by an increase in warm extremes and a decrease in cold extremes. The amount of water vapour (atmospheric humidity) in the atmosphere has very likely increased, consistent with the capacity of warmer air to hold more moisture. Not only has the ocean warmed at the surface, it is virtually certain that the whole upper ocean (to a depth of 700 m) has warmed. Global mean sea level has risen an estimated 0.19 m over the period 1901−2010 (90% uncertainty range between 0.17 m and 0.21 m) as a consequence of the expansion of ocean waters due to warming (warmer water takes up more volume) and the addition of new meltwater from shrinking glaciers and ice sheets worldwide. The extent of Arctic sea ice has also been shrinking in all seasons, with declines most evident in summer and autumn.

Understanding how much human activity has contributed to the observed warming of the climate system also draws from multiple lines of evidence. This includes evidence from observations, from improved understanding of processes and feedbacks within the system that determine how the climate system responds to both natural and human-induced perturbations, and from climate models (see Chapter 3.3.1).

The ability of greenhouse gases (GHGs) in Earth’s atmosphere to absorb heat energy radiated from the Earth is well understood. Emissions of GHGs from human activities have led to a build-up of atmospheric GHG levels. This rise in atmospheric GHG levels, predominantly carbon dioxide, has been the main driver of climate warming during the Industrial Era. The strong warming effect of increases in GHGs has been offset to some extent by increases in levels of atmospheric aerosols, which have climate-cooling effects. Variations in the brightness of the sun during the Industrial Era have had a warming effect on climate that is at least 10 times smaller than that from human activity and cannot explain the observed rise in global temperature. Volcanic eruptions have cooling effects on global climate that can last several years but cannot explain the observed long-term change in global temperature.

Determining how much of the observed climate warming and other climatic changes are due to these drivers is a complex task, as the climate system does not respond to these drivers in a straightforward way. To accomplish this task, climate (or Earth system) models are essential tools for identifying the causes of observed climate changes. Experiments with these models simulate how the climate system responds to real-world changes, including the impacts of human activities, and compare this with idealized experiments without human interference. On the basis of analysis of observations and such experiments, it is extremely likely that human influences, primarily emissions of GHGs, have been the dominant cause of the observed global warming since the mid-20th century. Studies have confirmed that there is a human contribution to observed changes in the lower atmosphere, the cryosphere, and the ocean, on a global scale.