Editing IPCC:Wg1:Chapter:Chapter-1-comments (section)

=== FAQ 1.1 | Do We Understand Climate Change Better Now Compared to When the IPCC Started? ===

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''Yes, much better. The first IPCC report, released in 1990, concluded that human-caused climate change would soon become evident, but could not yet confirm that it was already happening. Today, evidence is overwhelming that the climate has indeed changed since the pre-industrial era and that human activities are the principal cause of that change. With much more data and better models, we also understand more about how the atmosphere interacts with the ocean, ice, snow, ecosystems and land surfaces of the Earth. Computer climate simulations have also improved dramatically, incorporating many more natural processes and providing projections at much high'' ''er resolutions.''

Since the first IPCC report in 1990, large numbers of new instruments have been deployed to collect data in the air, on land, at sea and from outer space. These instruments measure temperature, clouds, winds, ice, snow, ocean currents, sea level, soot and dust in the air, and many other aspects of the climate system. New satellite instruments have also provided a wealth of increasingly fine-grained data. Additional data from older observing systems and even hand-written historical records are still being incorporated into observational datasets, and these datasets are now better integrated and adjusted for historical changes in instruments and measurement techniques. Ice cores, sediments, fossils, and other new evidence from the distant past have taught us much about how Earth’s climate has changed throughout its history.

Understanding of climate system processes has also improved. For example, in 1990 very little was known about how the deep ocean responds to climate change. Today, reconstructions of deep-ocean temperatures extend as far back as 1871. We now know that the oceans absorb most of the excess energy trapped by greenhouse gases and that even the deep ocean is warming up. As another example, in 1990, relatively little was known about exactly how or when the gigantic ice sheets of Greenland and Antarctica would respond to warming. Today, much more data and better models of ice-sheet behaviour reveal unexpectedly high melt rates that will lead to major changes within this century, including substantial sea level rise (FAQ 9.2).

The major natural factors contributing to climate change on time scales of decades to centuries are volcanic eruptions and variations in the sun’s energy output. Today, data show that changes in incoming solar energy since 1900 have contributed only slightly to global warming, and they exhibit a slight downward trend since the 1970s. Data also show that major volcanic eruptions have sometimes cooled the entire planet for relatively short periods of time (typically several years) by erupting aerosols (tiny airborne particles) high into the atmosphere.

The main human causes of climate change are the heat- absorbing greenhouse gases released by fossil fuel combustion, deforestation, and agriculture, which warm the planet; and aerosols such as sulphate from burning coal, which have a short-term cooling effect that partially counteracts human-caused warming. Since 1990, we have more and better observations of these human factors as well as improved historical records, resulting in more precise estimates of human influence on the climate sy stem (FAQ 3.1).

While most climate models in 1990 focused on the atmosphere, using highly simplified representations of oceans and land surfaces, today’s Earth system simulations include detailed models of oceans, ice, snow, vegetation and many other variables. An important test of models is their ability to simulate Earth’s climate over the period of instrumental records (since about 1850). Several rounds of such testing have taken place since 1990, and the testing itself has become much more rigorous and extensive. As a group and at large scales, models have predicted the observed changes well in these tests (FAQ 3.3). Since there is no way to do a controlled laboratory experiment on the actual Earth, climate model simulations can also provide a kind of ‘alternate Earth’ to test what would have happened without human influence. Such experiments show that the observed warming would not have occurred without human influence.

Finally, physical theory predicts that human influence on the climate system should produce specific patterns of change, and we see those patterns in both observations and climate simulations. For example, nights are warming faster than days, less heat is escaping to space, and the lower atmosphere (troposphere) is warming but the upper atmosphere (stratosphere) has cooled. These confirmed predictions are all evidence of changes driven primarily by increases in GHG concentrations rather than natural causes.

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'''FAQ 1.1, Figure 1 |''' '''Sample elements of climate understanding, observations and models as assessed in the IPCC First Assessment Report (1990) and Sixth Assessment Report (2021).''' Many other advances since 1990, such as key aspects of theoretical understanding, geological records and attribution of change to human influence, are not included in this figure because they are not readily represented in this simple format. Fuller explanations of the history of climate knowledge are available in the introductory chapters of the IPCC Fourth and Sixth assessment reports.
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