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

=== FAQ 2.1 | The Earth’s Temperature Has Varied Before. How Is the Current Warming Any Different? ===

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Earth’s climate has always changed naturally, but both the global extent and rate of recent warming are unusual. The recent warming has reversed a slow, long-term cooling trend, and research indicates that global surface temperature is higher now than it has been for millennia.

While climate can be characterized by many variables, temperature is a key indicator of the overall climate state, and global surface temperature is fundamental to characterizing and understanding global climate change, including Earth’s energy budget. A rich variety of geological evidence shows that temperature has changed throughout Earth’s history. A variety of natural archives from around the planet, such as ocean and lake sediments, glacier ice and tree rings, shows that there were times in the past when the planet was cooler, and times when it was warmer. While our confidence in quantifying large-scale temperature changes generally decreases the farther back in time we look, scientists can still identify at least four major differences between the recent warming and those of the past.

It’s warming almost everywhere. During decades and centuries of the past 2000 years, some regions warmed more than the global average while, at the same time, other regions cooled. For example, between the 10th and 13th centuries, the North Atlantic region warmed more than many other regions. In contrast, the pattern of recent surface warming is globally more uniform than for other decadal to centennial climate fluctuations over at least the past two millennia.

It’s warming rapidly. Over the past 2 million years, Earth’s climate has fluctuated between relatively warm interglacial periods and cooler glacial periods, when ice sheets grew over vast areas of the northern continents. Intervals of rapid warming coincided with the collapse of major ice sheets, heralding interglacial periods such as the present Holocene Epoch, which began about 12,000 years ago. During the shift from the last glacial period to the current interglacial, the total temperature increase was about 5°C. That change took about 5000 years, with a maximum warming rate of about 1.5°C per thousand years, although the transition was not smooth. In contrast, Earth’s surface has warmed approximately 1.1°C since 1850–1900. However, even the best reconstruction of global surface temperature during the last deglacial period is too coarsely resolved for direct comparison with a period as short as the past 150 years. But for the past 2000 years, we have higher-resolution records that show that the rate of global warming during the last 50 years has exceeded the rate of any other 50-year period.

Recent warming reversed a long-term global cooling trend. Following the last major glacial period, global surface temperature peaked by around 6500 years ago, then slowly cooled. The long-term cooling trend was punctuated by warmer decades and centuries. These fluctuations were minor compared with the persistent and prominent warming that began in the mid-19th century when the millennial-scale cooling trend was reversed.

It’s been a long time since it’s been this warm. Averaged over the globe, surface temperatures of the past decade were probably warmer than when the long cooling trend began around 6500 years ago. If so, we need to look back to at least the previous interglacial period, around 125,000 years ago, to find evidence for multi-centennial global surface temperatures that were warmer than now.

Previous temperature fluctuations were caused by large-scale natural processes, while the current warming is largely due to human causes (see, for example, FAQ 1.3, FAQ 3.1). But understanding how and why temperatures have changed in the past is critical for understanding the current warming and how human and natural influences will interact to determine what happens in the future. Studying past climate changes also makes it clear that, unlike previous climate changes, the effects of recent warming are occurring on top of stresses that make humans and nature vulnerable to changes in ways that they have never before experienced (for example, see FAQ 11.2, FAQ12.3).

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'''FAQ 2.1, Figure 1 |''' '''Evidence for the unusualness of recent warming.'''

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