Global temperature changes over the past million years
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Long-Term Global Temperature Trends Over the Past Million Years
Global temperatures have changed significantly over the past million years, continuing a broader trend of cooling that began several million years earlier. Reconstructions show that global temperature gradually cooled until about 1.2 million years ago, after which the cooling trend stalled and temperatures remained relatively stable until the present day. This cooling predated the increase in the size of ice sheets around 0.9 million years ago, suggesting that the drop in temperature may have set the stage for the development of large glacial cycles, but was not the only cause of these cycles Snyder2016Clark2024Ravelo2004.
Glacial Cycles and Periodicity in Temperature Change
During the Middle Pleistocene Transition (about 1.5 to 0.9 million years ago), there was a shift from 41,000-year glacial cycles with lower amplitude to 100,000-year cycles with higher amplitude. This change in the rhythm of glacial cycles was accompanied by accelerated cooling and is linked to changes in the carbon cycle, first driven by geological processes and later by changes in the Southern Ocean Clark2024Snyder2016. Over the last 800,000 years, global temperature and atmospheric greenhouse gas concentrations have been closely linked, with temperature changes at the poles (polar amplification) remaining stable .
Drivers of Global Temperature Change: Greenhouse Gases and Carbon Cycle
Greenhouse gases, especially carbon dioxide (CO2), have been the dominant driver of global temperature variations over the past million years and beyond. There is a strong correlation between atmospheric CO2 concentrations and global mean surface temperature, with Earth system sensitivity estimated at about 8–9°C per doubling of CO2 over millennial timescales Snyder2016Judd2024Li2023. This means that even if greenhouse gas levels stabilize at today’s values, Earth may still experience several degrees of warming over the next few millennia as slow feedbacks like ice sheet changes continue Snyder2016Judd2024.
Regional and Oceanic Influences on Climate
The transition from warmer to cooler global conditions over the past several million years was gradual, not the result of a single event. Regional differences in the timing of cooling, especially between high and low latitudes, suggest that changes in tropical and subtropical climates played a significant role in influencing global climate feedbacks Ravelo2004Zhang2024. Changes in sea surface temperatures (SSTs) and the gradient between equator and poles were closely tied to the development of large ice sheets and shifts in the carbon cycle, with SST changes often leading changes in ice volume and atmospheric CO2 .
Context of Recent and Future Warming
Current global temperatures are close to the warmest levels of the past million years, within about 1°C of the maximum during this period. Warming of more than 1°C above year 2000 levels is likely to have significant impacts, including sea level rise and species loss . The close coupling of temperature and greenhouse gases in the past highlights the potential for continued warming if greenhouse gas concentrations remain high Snyder2016Judd2024Hansen2006.
Conclusion
Over the past million years, global temperatures have been shaped by a combination of gradual cooling, changes in glacial cycle rhythms, and strong feedbacks involving greenhouse gases and the carbon cycle. Regional climate changes, especially in the oceans, have played a key role in these processes. The historical relationship between CO2 and temperature underscores the importance of greenhouse gases in driving long-term climate trends and provides important context for understanding current and future climate change.
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