They include:. There have been several hot periods during the Earth's past. Around 92 million years ago, for example, temperatures were so high that there were no polar ice caps and crocodile-like creatures lived as far north as the Canadian Arctic.
That should not comfort anyone, however, because humans were not around. At times in the past, sea level was 25m 80ft higher than the present. A rise of m ft is considered enough to submerge most of the world's coastal cities. There is abundant evidence for mass extinctions of life during these periods. And climate models suggest that, at times, the tropics could have become "dead zones" , too hot for most species to survive.
For many years, groups of so-called climate "sceptics" have cast doubt on the scientific basis of global warming. However, virtually all scientists who publish regularly in peer-reviewed journals now agree on the current causes of climate change.
A key UN report released in said it "is unequivocal that human influence has warmed the atmosphere, oceans and land". The COP26 global climate summit in Glasgow in November is seen as crucial if climate change is to be brought under control. Almost countries are being asked for their plans to cut emissions, and it could lead to major changes to our everyday lives. Top image credit: Getty Images. How do we know the world is getting warmer?
Image source, Reuters. Turkey was one of the places hit by devastating wildfires this summer. How do we know humans are responsible for global warming? Monitoring Greenhouse Gases in Antarctic Snow. Climate Change: Annual greenhouse gas index. Greenhouse Gas Concentrations - Graphing Tool.
Climate Forcing. Air - Atmospheric Climate Variables. Greenhouse Gases: A Closer Look. Catching a Heat Wave. Climate Change Indicators in the United States— Annual Greenhouse Gas Index. The magnitude of human and natural forcings will differ a bit between land and global temperatures.
For example, volcanic eruptions appear to have a larger influence on land, as land temperatures are likely to respond faster to rapid changes in forcings.
The figure below shows the relative contribution of each different radiative forcing to land temperatures since Land mean surface temperatures from Berkeley Earth black dots and modeled influence of different radiative forcings colored lines , as well as the combination of all forcings grey line for the period from to There is a wider variation in temperatures prior to , reflecting the much larger uncertainties in the observational records that far back.
There is still a period around and where observations exceed what the model predicts, though the differences are less pronounced than in global temperatures and the divergence is mostly absent in land records. Volcanic eruptions in the late s and early s stand out sharply in the land record. The eruption of Mount Tambora in Indonesia in may have cooled land temperatures by a massive 1. In general, volcanoes appear to cool land temperatures by nearly twice as much as global temperatures.
Carbon Brief used the same model to project future temperature changes associated with each forcing factor. The figure below shows observations up to , along with future post radiative forcings from RCP6.
Global mean surface temperatures from Berkeley Earth black dots and modeled influence of different radiative forcings colored lines for the period from to Forcings post taken from RCP6. When provided with the radiative forcings for the RCP6. Future radiative forcing from CO2 is expected to continue to increase if emissions rise.
This reduction in aerosols will enhance overall warming, bringing total warming from all radiative forcing closer to warming from greenhouse gases alone. The RCP scenarios assume no specific future volcanic eruptions, as the timing of these is unknowable, while solar output continues its year cycle. This approach can also be applied to land temperatures, as shown in the figure below. Here, land temperatures are shown between and , with post forcings also from RCP6. Land mean surface temperatures from Berkeley Earth black dots and modeled influence of different radiative forcings colored lines for the period from to This is seen in the model results, where land warms by around 4C by compared to 3C globally in the RCP6.
There is a wide range of future warming possible from different RCP scenarios and different values for the sensitivity of the climate system , but all show a similar pattern of declining future aerosol emissions and a larger role for greenhouse gas forcing in future temperatures.
While natural forcings from solar and volcanoes do not seem to play much of a role in long-term warming, there is also natural variability associated with ocean cycles and variations in ocean heat uptake. As the vast majority of energy trapped by greenhouse gases is absorbed by the oceans rather than the atmosphere, changes in the rate of ocean heat uptake can potentially have large impacts on the surface temperature.
While human factors explain all the long-term warming, there are some specific periods that appear to have warmed or cooled faster than can be explained based on our best estimates of radiative forcing. For example, the modest mismatch between the radiative forcing-based estimate and observations during the mids might be evidence of a role for natural variability during that period.
A number of researchers have examined the potential for natural variability to impact long-term warming trends. They have found that it generally plays a limited role. But that is a weak argument: you can, of course, never rule out the unknown unknown. The question is whether there is strong, or even any evidence for it. And the answer is no, in my view. Models get the short-term temperature variability approximately right. In many cases, they even have too much. But the forced response pretty much explains the observations, so there is no evidence from the 20th century that we are missing something….
Similarly, Dr Martin Stolpe and colleagues, also at IAC, recently analysed the role of multidecadal natural variability in both the Atlantic and Pacific oceans. Internal variability is likely to have a much larger role in regional temperatures. For example, in producing unusually warm periods in the Arctic and the US in the s. However, its role in influencing long-term changes in global surface temperatures appears to be limited.
The global warming witnessed over the past years matches nearly perfectly what is expected from greenhouse gas emissions and other human activity, both in the simple model examined here and in more complex climate models.
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