Editing IPCC:AR6/WGI/Chapter-2 (section)

=== FAQ 2.2 | What Is the Evidence for Climate Change? ===

<div id="h2-28-siblings" class="h2-siblings"></div>

<div id="faq-2-2"></div>

The evidence for climate change rests on more than just increasing surface temperatures. A broad range of indicators collectively leads to the inescapable conclusion that we are witnessing rapid changes to many aspects of our global climate. We are seeing changes in the atmosphere, ocean, cryosphere, and biosphere. Our scientific understanding depicts a coherent picture of a warming world.

We have long observed our changing climate. From the earliest scientists taking meteorological observations in the 16th and 17th centuries to the present, we have seen a revolution in our ability to observe and diagnose our changing climate. Today we can observe diverse aspects of our climate system from space, from aircraft and weather balloons, using a range of ground-based observing technologies, and using instruments that can measure to great depths in the ocean.

Observed changes in key indicators point to warming over land areas. Global surface temperature over land has increased since the late 19th century, and changes are apparent in a variety of societally relevant temperature extremes. Since the mid-1950s the troposphere (i.e., the lowest few km of the atmosphere) has warmed, and precipitation over land has increased. Near-surface specific humidity (i.e., water vapour) over land has increased since at least the 1970s. Aspects of atmospheric circulation have also evolved since the mid-20th century, including a poleward shift of mid-latitude storm tracks.

Changes in the global ocean point to warming as well. Global average sea surface temperature has increased since the late 19th century. The heat content of the global ocean has increased since the 19th century, with more than 90% of the excess energy accumulated in the climate system being stored in the ocean. This ocean warming has caused ocean waters to expand, which has contributed to the increase in global sea level in the past century. The relative acidity of the ocean has also increased since the early 20th century, caused by the uptake of carbon dioxide from the atmosphere, and oxygen loss is evident in the upper ocean since the 1970s.

Significant changes are also evident over the cryosphere – the portion of the Earth where water is seasonally or continuously frozen as snow or ice. There have been decreases in Arctic sea ice area and thickness and changes in Antarctic sea ice extent since the mid-1970s. Spring snow cover in the Northern Hemisphere has decreased since the late-1970s, along with an observed warming and thawing of permafrost (perennially frozen ground). The Greenland and Antarctic ice sheets are shrinking, as are the vast majority of glaciers worldwide, contributing strongly to the observed sea level rise.

Many aspects of the biosphere are also changing. Over the last century, long-term ecological surveys show that many land species have generally moved poleward and to higher elevations. There have been increases in green leaf area and/or mass (i.e., global greenness) since the early 1980s, and the length of the growing season has increased over much of the extratropical Northern Hemisphere since at least the mid-20th century. There is also strong evidence that various phenological metrics (such as the timing of fish migrations) for many marine species have changed in the last half century.

Change is apparent across many components of the climate system. It has been observed using a very broad range of techniques and analysed independently by numerous groups around the world. The changes are consistent in pointing to a climate system that has undergone rapid warming since the industrial revolution.

[[File:b7cd6ec4241444bf5ca4ac3066a2dd8c IPCC_AR6_WGI_FAQ_2_2_Figure_1.png]]
'''FAQ 2.2, Figure 1 |''' '''Synthesis of significant changes observed in the climate system over the past several decades.''' Upwards, downwards and circling arrows indicate increases, decreases and changes, respectively. Independent analyses of many components of the climate system that would be expected to change in a warming world exhibit trends consistent with warming. Note that this list is not comprehensive.

<div id="references" class="h1-container"></div>