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Predictions of Future Global Climate

Temperature

Temperatures are, of course, expected to rise in the future. "By how much?" depends on the assumed emissions scenario as well as the projections based on different climate models.

The six emission scenarios used by the IPCC in the Fourth Assessment Report yield the following projections (all increases are in degrees Celsius as compared to the average for 1980 to 1999) for temperatures in the 2090 to 2099 timeframe:

Scenario
Best estimate
temperature increase
Likely minimum
increase
Likely maximum
increase
B1
1.8
1.1
2.9
A1T
2.4
1.4
3.8
B2
2.4
1.4
3.8
A1B
2.8
1.7
4.4
A2
3.4
2.0
5.4
A1FI
4.0
2.4
6.4

In graphical form, these data look like this:

Global temperatures - future

Projections of future global average surface temperature for various IPCC scenarios. The graph shows temperature changes (as compared with the 1980-1999 average, which is used as the baseline) for three scenarios (A2, A1B, and B1). Solid colored lines represent "most likely" trends; shaded regions represent "probable ranges". The gray bars on the right represent year 2100 temperatures for all six scenarios; the colored stripe represents the "best estimate", while the shaded gray region represents "likely ranges". The different scenarios and models predict temperature changes between one and slightly more than six degrees Celsius.

Temperature increases are expected to be greater on land than over oceans. Temperature increases are expected to be greater at high latitudes than in the tropics and mid-latitudes. Temperature increase expected through about 2025 is largely independent from the scenario assumed; after that, the scenario choice plays a growing role in the expected temperature rise.

"It is very likely that heat waves will be more intense, more frequent and longer lasting in a future warmer climate. Cold episodes are projected to decrease significantly in a future warmer climate. Almost everywhere, daily minimum temperatures are projected to increase faster than daily maximum temperatures, leading to a decrease in diurnal temperature range. Decreases in frost days are projected to occur almost everywhere in the middle and high latitudes, with a comparable increase in growing season length." - IPCC AR4 WG1 Chapter 10 page 750

Precipitation

Higher average global temperatures will cause a higher overall rate of evaporation. More water vapor in the atmosphere will drive higher overall rates of precipitation. The global water cycle will "speed up", so more water will be sucked up into the atmosphere and then proceed to fall back out as rain and snow.

Changes in precipitation will vary by time and location. Some locations will get more snow, others will see less rain. Some places will have wetter winters and drier summers, and so on.

Models indicate that extreme precipitation will occur more frequently. Drenching downpours will generally be more prevalent. Some regions predicted to experience overall declines in average annual precipitation will actually be hit with short periods of more intense rainfall followed by longer periods between rain. There is a tendency for drying of the mid-continental areas during summer, indicating a greater risk of droughts in those regions.

Projected precipitation changesProjected changes in global total precipitation through the year 2100. All three graphs show projections from several different climate models (colored lines) and a sort of average for all of the models (dotted black line). The three graphs represent three different IPCC scenarios; scenario A2 (top), A1B (middle), and B1 (bottom). All values are relative to the 1980 to 1999 average.

The "average" (black line) precipitation increase by 2100 is projected across the three scenarios to be between about 3% and 5%. One model in the A2 scenario puts it as high as 8%; one in the A1B scenario projects just a 1% increase.

Credits: Image courtesy of the IPCC (AR4 WG1 Chapter 10 page 763 Figure 10.5).

Predicted precipitation mapsProjected changes in precipitation in 2100. Blue and green areas are expected to experience increases in precipitation. Yellow and pink areas are projected to have decreases.

The top image shows the period covering the months of December, January, and February. The bottom image covers June, July, and August.

Most high-latitude regions experience increased precipitation in both winter and summer. Most land areas, except for much of Asia, Greenland, and northern North America, are drier in the June through August timeframe. The pattern of precipitation on land in December through February is much more of a patchwork of mixed drier and moister regions.

Credits: Image courtesy of the IPCC (AR4 WG1 FAQ 11.1 page 127 Figure 1).

Snow and Ice

"As the climate warms, snow cover and sea ice extent decrease; glaciers and ice caps lose mass owing to a dominance of summer melting over winter precipitation increases. This contributes to sea level rise as documented for the previous generation of models in the TAR. There is a projected reduction of sea ice in the 21st century in both the Arctic and Antarctic with a rather large range of model responses. The projected reduction is accelerated in the Arctic, where some models project summer sea ice cover to disappear entirely in the high-emission A2 scenario in the latter part of the 21st century. Widespread increases in thaw depth over much of the permafrost regions are projected to occur in response to warming over the next century." - IPCC AR4 WG1 Chapter 10 page 750

AlbedoDecreased snow and ice cover tends to accelerate warming in a positive feedback loop. Either land or sea beneath ice or snow tends to have a much lower albedo than the overlaying frosty covering. Once the ice or snow is removed, the lower-albedo surface absorbs more incoming sunlight, warming it further and thus melting more ice and snow.

Melting of the vast tracks of permafrost in the Arctic tundra may also produce a positive feedback reinforcement to the warming trend. Melting permafrost may release vast quantities of methane, also a powerful greenhouse gas, into Earth's atmosphere. The resultant increase in the greenhouse effect figures to exert further warming pressures on our planet's climate.

Some regions, especially near the Tibetan Plateau in Asia and along the Andes Mountains in South America, rely heavily on glacial meltwater for their water supplies for both human consumption and for agriculture. Mountain glaciers are receding and even disappearing in many places, leaving nearby communities that are dependent on them as water sources with uncertain futures.

Melting sea ice in the Arctic is already causing various changes at high latitudes. Creatures such as polar bears, for which the sea ice pack is an integral part of the environment, are struggling to adapt to decreased ice packs. New shipping routes, especially the fabled "Northwest Passage" through the islands of northern Canada, may open up in the summer. Native peoples at high northern latitudes are finding that aspects of their traditional lifestyles are being disrupted by changes in the ice pack. The IPCC predicts continuing decreases in sea ice in both hemispheres; some projections forecast the complete loss of Arctic sea ice in the summertime by the late 21st century.

Sea Level

A warmer climate pushes sea levels higher via two mechanisms. Melting glaciers and ice sheets (but not floating sea ice!) add water to the oceans, raising sea level. As ocean water warms it also expands, displacing a greater volume and thus also raising sea levels.

During the 20th century, sea levels rose about 10 to 20 cm (4 to 8 inches). Thermal expansion and melting ice each contributed about half of the rise, though there is a fair amount of uncertainty in the exact magnitude of the contribution from each source.

The various IPCC emission scenarios and climate models produce a range of expected future sea level changes. By the year 2100, those models predict sea level will rise between about 20 and 50 cm (8 to 20 inches) above late 20th century levels. Thermal expansion of sea water is predicted to account for about 75% of future sea level rise according to most models.

Sea Level Rise to date

Sea level data from 1880 to the present, as measured by tide gages and satellite altimetry. The numerous colored lines indicate records from each of the 23 tide gages; the thicker black curve is a running 3-year average of all gages combined. The shorter thick red line shows recent data from satellite measurements.

Credit: Image by Robert A. Rohde, courtesy of the Global Warming Art Project.

Predicted future sea level rise

Predicted future sea level rise, based on a range of IPCC scenarios and climate models. Sea level is expected to rise between 20 to 50 centimeters in the 21st century. The blue shaded region on the graph shows the range of values generated by various scenarios and models.

Credits: Image courtesy of the IPCC (AR4 WG1 FAQ 5.1 page 111 Figure 1).

Higher sea levels threaten coastal communities in various ways, and are problematic because so much of the world's population lives close to a coast. Higher seas increase flooding risks and speed up beach erosion. They threaten some islands with complete inundation, potentially wiping them from the map entirely. Storm surges that accompany hurricanes will already be riding higher atop raised seas, so their flooding will slosh further inland. Some brackish coastal ecosystems will experience changes in salinity, altering the types of organisms that can survive in those regions.

Sea level rise is a "tipping point" issue. Some changes to climate are expected to occur gradually in a regular, relatively predictable fashion. Others have the potential for abrupt shifts that are much harder to predict. Warming climates could cause portions of either the Greenland or Antarctic ice sheets to become unstable and melt much more quickly than most models account for. Such plausible but not easily predictable events could cause sea levels to rise much more than is currently forecast. The IPCC projections of sea level do not include dramatic shifts because they are so difficult to predict; but the IPCC reports do acknowledge the significant possibility of such meltdowns occurring.

The Oceans - other changes

Hurricane KatrinaEarth's oceans act as a buffer against climate change in various ways. The high thermal inertia of water prevents the oceans from warming as quickly as the atmosphere. This is good news in the short run, but more problematic in the long run; much of the warming we inflict on our home world will gradually be stored away in the oceans, and cooling that water if/when we do get a handle on global warming will take many centuries. The oceans also absorb a lot of carbon dioxide from the atmosphere, preventing at least some CO2 emissions from contributing to greenhouse warming.

Carbon dioxide can combine with water to form weak carbonic acid. When ocean water absorbs CO2 its pH decreases, making it more acidic. This may cause problems for marine organisms and certainly alters oceanic chemistry. Scientists believe the pH of the oceans has dropped about 0.1 pH since pre-industrial times. Further acidification of 0.14 to 0.35 pH is expected, depending on the emission scenario and climate model employed, by the year 2100.

Deep water currents in the oceans are driven by density differences in sea water. These density variations are a result of differences of temperature and salinity of various "parcels" of water. Changes in precipitation patterns and the influx of fresh water into the oceans from increased glacial and ice sheet melting can alter salinity. Such changes, along with alterations in water temperature, may disrupt the current circulation systems. In an extreme case, the great thermohaline circulation systems in the oceans could be severely disrupted or even shut down in some parts of the ocean. Such an occurrence could have large and unpredictable effects on climate.

Some climate scientists believe that hurricanes, typhoons, and other tropical cyclones will (and may have begun to already) change as a result of global warming. Warm ocean surface waters provide the energy that drives these immense storms. Warmer oceans in the future are expected to cause intensification of such storms. Although there may not be more tropical cyclones worldwide in the future, some scientists believe there will be a higher proportion of the most powerful and destructive storms. Tropical storms may produce increased flooding via heavier rainfall rates and because of increased danger from storm surges due to higher global sea levels. Some scientists believe we are already seeing evidence for an upswing in the numbers of the most powerful storms; others are less convinced.

Clouds

CloudsClouds are a bit of a wild-card in global climate models. Warmer global temperatures produce faster overall evaporation rates, resulting in more water vapor in the atmosphere... and hence more clouds. Different types of clouds (cirrus, cumulus, etc.) at different heights in the atmosphere and in different latitude ranges (polar, tropical, etc.) have different effects on climate. Clouds can cool the climate by increasing albedo; they can also warm things up via the enhanced greenhouse effect produced by the water vapor and droplets within them. Different types of clouds have different mixes of these two effects. Scientists expect a warmer world to be a cloudier one, but are not yet certain how the increased cloudiness will feed back into the climate system. Modeling the influence of clouds in the climate system is an area of active scientific research.

Biological Systems and the Carbon Cycle

Climate change will alter many aspects of biological systems and the global carbon cycle. Temperature changes will alter the natural ranges of many types of plants and animals, both wild and domesticated. The lengths of growing seasons, geographical ranges throughout which plants can survive, and dates of frosts will change with the warming climate. This will impact both agriculture and wild species of plants. Some invasive species will be able to move into new regions. Certain insect-borne diseases, such as malaria, will spread beyond their traditional tropical boundaries.

Modeling of the global carbon cycle suggest that the overall Earth system will be able to absorb less CO2 out of the atmosphere as the climate warms. As our planet heats up, it will apparently lose some of its capacity to help "sponge up" excess CO2 from the atmosphere, worsening the warming problem.

Climate Commitment and DAI

"Commitment" and "DAI" are two concepts you may encounter when reading about predictions of future climate.

In climate science, "commitment" expresses the fact that many systems have a sort of momentum that will continue to carry them in a certain direction no matter what we do to change them today. Because excess carbon dioxide leaves the atmosphere very, very slowly, past CO2 emissions have committed us to a certain minimum level of CO2 concentration in the atmosphere for many decades to come, even if we completely eliminated new emissions this year. Likewise, global temperatures would continue to rise (by an estimated 0.5° C), even if greenhouse gas emissions were halted immediately, because of the the amounts of these gases already present in the atmosphere. We are likewise committed to a certain level of ocean warming (as heat is slowly transferred to water from air) and to a certain amount of sea ice melting (as warmer ocean temperatures and lowered albedo from lost snow cover combine to melt the ice) in the future. These effects are minimums based on what has already happened; they can, of course, be made larger by continued emissions of greenhouse gases.

DAI is an abbreviation for Dangerous Anthropogenic Interference. International efforts to curb the worst aspects of climate change use the notion of DAI when countries try to come to agreement on what actions, over what timeframe, are needed to prevent "severe" problems associated with changing climate. DAI is definitely not a clearly-defined, well-established standard; it is much more of a general concept. Rising temperatures, increased incidence of severe droughts, and loss of water supplies by people dependent on glacial runoff are all potentially dangerous problems. Discussions, often contentious, between participants at climate change mitigation meetings often focus on how severe a problem must be to be considered "dangerous", and what changes to human behavior (such as CO2 emission levels) would minimally be required to avoid DAI.

Possible abrupt changes & tipping points

As mentioned in some of the topics above, some changes to climate are gradual and predictable, while others are more sudden and difficult to foresee. The later category of effects are often referred to as "tipping point" issues; ones that can swing abruptly into a large change that cannot readily be "backed out of" at the last minute even by employing drastic measures.

Collapse of major ice sheets in Greenland and Antarctica is one such tipping point issue. Melting of these ice sheets is an ongoing process; however, there are signs that moderate melting may accelerate into a runaway situation that leads to a relatively sudden loss of large amounts of ice. Such a collapse could lead to dramatic changes in sea level, and could also impact ocean circulation.

Disruption of the thermohaline "conveyor belt" that drives major ocean circulation patterns, especially in the deep oceans, could also be a "tipping point" phenomenon. Changes in ocean temperature and salinity could disrupt the density differences that drive this circulation. If the thermohaline circulation changed dramatically or even shut down altogether, the transfer of heat in the climate system would be altered in a huge way. For example, the warming effects of the Gulf Stream on the Atlantic seaboard of North America and on much of Europe could vanish, substantially changing the climate in those regions.

Other "tipping point" phenomena involve massive, sudden releases of stored reservoirs of methane. Methane is a very potent greenhouse gas. Scientists believe that huge quantities of methane are trapped in special ices, called "methane hydrates" or "clathrates", which form in sediments beneath the sea floor. Rising ocean temperatures might melt some of these ices, releasing some of the methane; this would generate a positive feedback cycle in which the increased greenhouse warming of the additional methane in the atmosphere would warm the air, which would warm the oceans, which would melt more clathrates, triggering yet more methane emissions. "Natural" methane emissions might also suddenly rise if large areas of permafrost melt. The microbiologically active mush that would be created from melting permafrost is expected to release lots of methane, again generating a feedback loop of increased greenhouse warming by methane driving further methane emissions. Some scientists suspect that sudden increases in methane emissions may have played a role in major extinction events in the past.

A fourth tipping point issue involves absorption of CO2 by the oceans. As mentioned earlier, sea water that absorbs carbon dioxide becomes increasingly acidic as the CO2 combines with water to form carbonic acid. At some point the water becomes saturated and cannot absorb any further carbon dioxide. Right now, the oceans are helping mitigate greenhouse warming by absorbing much of the CO2 that would otherwise stay in the atmosphere. If anthropogenic emissions of CO2 continue to rise even as the oceans become saturated, the atmospheric concentration of carbon dioxide could rise much more suddenly than it has been, causing increased greenhouse warming. Acidification of the oceans could also disrupt ocean chemistry to the point that major biological systems might suddenly be in turn disrupted. Vast swarms of photosynthesizing plankton might succumb, preventing them from removing CO2 from the air. Shells of many types of marine organisms might begin to dissolve in the presence of the acidic oceans, releasing the carbon stored within the shells back into the environment.

None of these tipping point issues are considered very likely to occur in the short run; but the consequences of any of them being set off are so severe, and the fact that we cannot retreat from them once they've been set in motion is so problematic, that we must keep them in mind when evaluating the overall risks associated with different levels of climate change.

Last modified April 14, 2008 by Randy Russell.

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