Based on the results of climate models, increases in mean global warming from present-day to 1.5°C over pre-industrial levels will produce substantial changes in many climate systems, including rising mean temperature in most land and ocean regions. These effects will also result in increases in precipitation and drought extremes and changes in many other climate issues. These changes will be greatly exacerbated by global warming to 2°C.

For global warming to 1.5°C above pre-industrial levels, extremely high temperatures is expected in many regions. This is accompanied by expected increases in frequency and intensity of both heavy precipitation and drought. Temperature extremes on land in mid-latitudes are expected to warm during the day by about 3°C for 1.5°C over pre-industrial global warming and about 4°C for 2°C over pre-industrial global warming. At night in high latitudes these temperatures are projected to be 4.5°C at 1.5°C and 6°C at 2°C over pre-industrial global warming.

Increases in extremely heavy precipitation are expected for global warming levels of 1.5°C over pre-industrial levels. In addition, for 2°C global warming levels increases of extreme precipitation are expected to be much higher than at 1.5°C. This is especially the case in high-latitude and high elevation areas and in eastern Asia and eastern North America. Extremely higher degree and frequency of flooding are also expected.

Increases in extreme drought conditions are expected for 1.5°C global warming levels. In addition, for 2°C global warming levels increases of drought are expected to be much higher than at 1.5°C.

Relative to sea level at 1986-2005, models project a global sea level rise of 0.26-0.77 meters by the year 2100 for 1.5°C global warming. For 2°C global warming, sea level rise is projected to be approximately 0.1 meters more. That sea level rise of an additional 0.1 meters would expose approximately 10 million more people to sea rise-related risks. These estimates are based on the projected global population in 2100 and no adaptation efforts.

Even if global warming is limited to 1.5°C, sea level rise will continue beyond 2100. Furthermore, at global warming of 1.5°C or 2°C, the Greenland and Antarctica ice sheets could reach a pivotal level of instability that would take their melting to the point of adding multiple meters of sea rise over hundreds or thousands of years.

Small islands, low-lying coastal areas and deltas are particularly vulnerable to the effects of sea level rise, including saltwater intrusion, flooding, and damage to infrastructure. These effects are felt at global warming of 1.5°C, but they become more damaging if global warming extends to 2°C.

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A study of 105,000 species was conducted to determine the effect of global warming on their climatically determined geographic range. At 1.5°C global warming, 6% of insects, 8% of plants and 4% of vertebrates were projected to lose over half of their geographic range. At 2°C global warming, 18% of insects, 16% of plants and 8% of vertebrates were projected to lose over half of their geographic range.

When considering the effect of global temperature on how land is used by ecosystems, about 4% of the global terrestrial land area is projected to undergo a transformation of ecosystems from one type to another at 1°C of global warming. At 2°C global warming, that percentage changes from 4% to approximately 13%. From these statistics it can be estimated that at 1.5°C, the ecosystem transformation occurs at a rate of about 7%.

Finally, the land most vulnerable to global warming are high-altitude tundra and boreal forests environments.

In polar regions, especially in the Arctic regions, the land has for centuries been permanently hardened by freezing. This hardened soil is referred to as permafrost. One of its characteristics is that it contains a sizable amount of methane hydrates, a form of frozen water that holds molecules of methane within a crystalline structure.

When permafrost is thawed, particularly by global warming, it releases the methane (or other greenhouse gases) that has been held in its crystalline structure. If global warming increases to 2°C, instead of leveling off at 1.5°C, it is possible that the release of methane could initiate a positive feedback process that would cause thawing of permafrost to extend over centuries, covering an area in the range of 1.5 to 2.5 million square kilometers.

There is a growing probability that 1.5°C global warming will result in the Arctic Ocean being free of sea ice during the summer at least once every century. If global warming increases to 2°C, this eventuality increases in probability and extent to there being an sea ice-free summer once per decade.

When carbon dioxide dissolves in sea water, it forms carbonic acid, thereby increasing the acidity of the oceans. This effect impacts the growth, development, calcification, survival and thus abundance of a broad range of species, for example, from algae to fish. An increase in global warming of 1.5°C will produce less acidity than an increase of 2°C, because the higher the warming, the more carbon dioxide is available to dissolve in the ocean, thereby increasing acidity.

Populations at greatest risk of adverse consequences from global warming include disadvantaged and vulnerable populations, some indigenous peoples, and communities dependent on agricultural or coastal livelihoods. Regions at greatest risk include Arctic ecosystems, dryland regions, small island developing states, and Least Developed Countries. If global warming is limited to 1.5°C, instead of 2°C, the number of people exposed to these intense negative effects could be reduced by up to several hundred million by 2050.

Negative consequences of excessive global warming on human health include:

• heat-related morbidity and mortality
• ozone-related mortality
• vector-borne diseases (e.g. malaria and dengue fever)
• reduction in crop yields, particularly cereal crops
• livestock vulnerability to heat
• lack of availability of water

Global economic growth and stability is expected to be reduced by global warming, to a greater extent by 2°C warming than by 1.5°C warming. The largest economic impacts will be felt by countries in the tropics and Southern Hemisphere subtropics.

Climate change adaptation involves adjustment to the new conditions imposed by the effects of climate change. The amount of adaption increases with the degree of change, and that is determined to a large extent by the amount of global warming that has occurred. Thus, 1.5°C global warming will require less adaptation than 2°C global warming.

There are a wide range of adaptation methods available to adjust to the many changes that will be required. Following is a list of the areas of risk and potential negative impact where adaptation measures may be necessary:

• ecosystem adaptation
• ecosystem restoration
• degradation and deforestation
• sustainable aquaculture
• sea level rise
• risks to health, livelihoods, food, water, economic growth
• sustainable land use and water management
• and many more ...

The most vulnerable regions include small islands, Least Developed Countries, and areas of poverty and economic stress. These environments will likely experience high multiple interrelated climate risks, which will increase in intensity as global warming levels rise.