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

=== FAQ 1.2 | Where Is Climate Change Most Apparent? ===

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''The signs of climate change are unequivocal at the global scale and are increasingly apparent on smaller spatial scales. The high northern latitudes show the largest temperature increase, with clear effects on sea ice and glaciers. The warming in the tropical regions is also apparent because the natural year-to-year variations in temperature there are small. Long-term changes in other variables such as rainfall and some weather and climate extremes have also now become apparent i'' ''n many regions.''

It was first noticed that the planet’s land areas were warming in the 1930s. Although increasing atmospheric carbon dioxide (CO <sub>2</sub> ) concentrations were suggested as part of the explanation, it was not certain at the time whether the observed warming was part of a long-term trend or a natural fluctuation: global warming had not yet become apparent. But the planet continued to warm, and by the 1980s the changes in temperature had become obvious or, in other words, the ''sign'' ''al'' had ''emerged'' .

Imagine you had been monitoring temperatures at the same location for the past 150 years. What would you have experienced? When would the warming have become noticeable in your data? The answers to these questions depend on where on the planet you are.

Observations and climate model simulations both demonstrate that the largest long-term warming trends are in the high northern latitudes and the smallest warming trends over land are in tropical regions. However, the year-to-year variations in temperature are smallest in the tropics, meaning that the changes there are also apparent, relative to the range of past experiences (FAQ 1.2, Figure 1).

Changes in temperature also tend to be more apparent over land areas than over the open ocean and are often most apparent in regions which are more vulnerable to climate change. It is expected that future changes will continue to show the largest signals at high northern latitudes, but with the most apparent warming in the tropics. The tropics also stand to benefit the most from climate change mitigation in this context, as limiting global warming will also limit how far the climate shifts relative to past experience.

Changes in other climate variables have also become apparent at smaller spatial scales. For example, changes in average rainfall are becoming clear in some regions, but not in others, mainly because natural year-to-year variations in precipitation tend to be large relative to the magnitude of the long-term trends. However, extreme rainfall is becoming more intense in many regions, potentially increasing the impacts from inland flooding (FAQ 8.2). Sea levels are also clearly rising on many coastlines, increasing the impacts of inundation from coastal storm surges, even without any increase in the number of storms reaching land. A decline in the amount of Arctic sea ice is apparent, both in the area covered and in its thickness, with implications for polar ecosystems.

When considering climate-related impacts, it is not necessarily the size of the change that is most important. Instead, it can be the rate of change or it can also be the size of the change relative to the natural variations of the climate to which ecosystems and society are adapted. As the climate is pushed further away from past experiences and enters an unprecedented state, the impacts can become larger, along with the challenge of adapting to them.

How and when a long-term trend becomes distinguishable from shorter-term natural variations depends on the aspect of climate being considered (e.g., temperature, rainfall, sea ice or sea level), the region being considered, the rate of change, and the magnitude and timing of natural variations. When assessing the local impacts from climate change, both the size of the change and the amplitude of natural variations matter.

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'''FAQ 1.2, Figure 1''' | '''Observed variations in regional temperatures since 1850''' (data from Berkeley Earth). Regions in high latitudes, such as mid-North America (40°N–64°N, 140°W–60°W, '''left''' ), have warmed by a larger amount than regions at lower latitudes, such as tropical South America (10°S–10°N, 84°W–16°W, '''right''' ), but the natural variations are also much larger at high latitudes (darker and lighter shading represents 1 and 2 standard deviations, respectively, of natural year-to-year variations). The signal of observed temperature change emerged earlier in tropical South America than mid-North America even though the changes were of a smaller magnitude. (Note that those regions were chosen because of the longer length of their observational record; see Figure 1.14 for more regions).
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