Editing IPCC:AR6/WGI/Chapter-1 (section)

=== FAQ 1.3 | What Can Past Climate Teach Us About the Future? ===

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''In the past, the Earth has experienced prolonged periods of elevated greenhouse gas concentrations that caused global temperatures and sea levels to rise. Studying these past warm periods informs us about the potential long-term consequences of increasing greenhouse gases in'' ''the atmosphere.''

Rising greenhouse gas concentrations are driving profound changes to the Earth system, including global warming, sea level rise, increases in climate and weather extremes, ocean acidification, and ecological shifts (FAQ 2.2 and FAQ 7.1). The vast majority of instrumental observations of climate began during the 20th century, when greenhouse gas emissions from human activities became the dominant driver of changes in Earth’s climate (FAQ 3.1).

As scientists seek to refine our understanding of Earth’s climate system and how it may evolve in coming decades to centuries, past climate states provide a wealth of insights. Data about these past states help to establish the relationship between natural climate drivers and the history of changes in global temperature, global sea levels, the carbon cycle, ocean circulation, and regional climate patterns, including climate extremes. Guided by such data, scientists use Earth system models to identify the chain of events underlying the transitions between past climatic states (FAQ 3.3). This is important because during present-day climate change, just as in past climate changes, some aspects of the Earth system (e.g., surface temperature) respond to changes in greenhouse gases on a time scale of decades to centuries, while others (e.g., sea level and the carbon cycle) respond over centuries to millennia (FAQ 5.3). In this way, past climate states serve as critical benchmarks for climate model simulations, improving our understanding of the sequences, rates, and magnitude of future climate change over the next decades to millennia.

Analyzing previous warm periods caused by natural factors can help us understand how key aspects of the climate system evolve in response to warming. For example, one previous warm-climate state occurred roughly 125,000 years ago, during the Last Interglacial period, when slight variations in the Earth’s orbit triggered a sequence of changes that caused about 1°C–2°C of global warming and about 2–8 m of sea level rise relative to the 1850–1900, even though atmospheric carbon dioxide concentrations were similar to 1850–1900 values (FAQ 1.3, Figure 1). Modelling studies highlight that increased summer heating in the higher latitudes of the Northern Hemisphere during this time caused widespread melting of snow and ice, reducing the reflectivity of the planet and increasing the absorption of solar energy by the Earth’s surface. This gave rise to global-scale warming, which led in turn to further ice loss and sea level rise. These self-reinforcing positive ''feedback'' ''cycles'' are a pervasive feature of Earth’s climate system, with clear implications for future climate change under continued greenhouse gas emissions. In the case of sea level rise, these cycles evolved over several centuries to millennia, reminding us that the rates and magnitude of sea level rise in the 21st century are just a fraction of the sea level rise that will ultimately occur after the Earth system fully adjusts to current levels of global warming.

Roughly 3 million years ago, during the Pliocene Epoch, the Earth witnessed a prolonged period of elevated temperatures (2.5°C–4°C higher than 1850–1900) and higher sea levels (5–25 m higher than 1850–1900), in combination with atmospheric carbon dioxide concentrations similar to those of the present day. The fact that Pliocene atmospheric carbon dioxide concentrations were similar to the present, while global temperatures and sea levels were significantly higher, reflects the difference between an Earth system that has fully adjusted to changes in natural drivers (the Pliocene) and one where greenhouse gases concentrations, temperature, and sea level rise are still increasing (present day). Much about the transition into the Pliocene climate state – in terms of key causes, the role of cycles that hastened or slowed the transition, and the rate of change in climate indicators such as sea level – remain topics of intense study by climate researchers, using a combination of paleoclimate observations and Earth system models. Insights from such studies may help to reduce the large uncertainties around estimates of global sea level rise by 2300, which range from 0.3 m to 3 m above 1850–1900 (in a low-emissions scenario) to as much as 16 m higher than 1850–1900 (in a very high-emissions scenario that includes accelerating structural disintegration of the polar ice sheets).

While present-day warming is unusual in the context of the recent geologic past in several different ways (FAQ 2.1), past warm climate states present a stark reminder that the long-term adjustment to present-day atmospheric carbon dioxide concentrations has only just begun. That adjustment will continue over the coming centuries to millennia.

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'''FAQ 1.3, Figure 1 |''' '''Comparison of past, present and future.''' Schematic of atmospheric carbon dioxide concentrations, global temperature, and global sea level during previous warm periods as compared to 1850–1900, present-day (2011–2020), and future (2100) climate change scenarios corresponding to low-emissions scenarios (SSP1-2.6; lighter colour bars) and very high-emissions scenarios (SSP5-8.5; darker colour bars).

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