Temperature Will Rise: Climate models predict that Earth’s global average temperate will rise in the future. For the next two decades warming of about 0.2° Celsius is projected. If we continue to emit as many, or more, greenhouse gases, this will cause more warming during the 21st Century than we saw in the 20th Century. During the 21st Century, various computer models predict that Earth’s average temperature will rise between 1.8° and 4.0° Celsius (3.2° and 7.2° F). The amount of predicted warming differs depending on the model emissions scenario (how much greenhouse gas emissions it assumes for the future). The amount of predicted warming also differs between different climate models. Climate change is predicted to impact regions differently. For example, temperature increases are expected to be greater on land than over oceans and greater at high latitudes than in the tropics and mid-latitudes.
Changing Precipitation: Warmer average global temperature will cause a higher rate of evaporation, causing the water cycle to “speed up”. More water vapor in the atmosphere will lead to more precipitation. According to models, global average precipitation will most likely increase by about 3-5% with a minimum increase of at least 1% and a maximum increase of about 8%. Yet, changes in precipitation will not be evenly distributed. Some locations will get more snow, others will see less rain. Some places will have wetter winters and drier summers.
Melting Snow and Ice: As the climate warms, snow and ice melt. The amount of summer melting of glaciers, ice sheets, and other snow and ice on land are predicted to be greater than the amount of winter precipitation. The amount of sea ice (frozen seawater) floating in the ocean in the Arctic and Antarctic is expected to decrease over the 21st Century too, although there is some uncertainty as to the amount of melt.
Rising Sea Level: A warmer climate causes sea level to rise via two mechanisms: (1) melting glaciers and ice sheets (ice on land) add water to the oceans, raising sea level, and (2) ocean water expands as it warms, increasing its volume and thus also raising sea level. During the 20th Century, sea level 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 some uncertainty in the exact magnitude of the contribution from each source. By the year 2100, 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.
Acidic Ocean Water: Earth's oceans are predicted to act as a buffer against climate change by taking up some of the excess heat and carbon dioxide from the atmosphere. This is good news in the short run, but more problematic in the long run. Carbon dioxide combined with seawater forms weak carbonic acid. Scientists believe this process has reduced the pH of the oceans by about 0.1 pH since pre-industrial times. Further acidification of 0.14 to 0.35 pH is expected by the year 2100. More acidic ocean water may cause problems for marine organisms.
Impacts on Ocean Currents: Large-scale ocean currents called thermohaline circulation, driven by differences in salinity and temperature, may also be disrupted as climate warms. Changes in precipitation patterns and the influx of fresh water into the oceans from melting ice can alter salinity. Changing salinity, along with rising water temperature, may disrupt the currents. In an extreme case, thermohaline circulation could be disrupted or even shut down in some parts of the ocean, which could have large effects on climate.
Changing Severe Weather: 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. Some scientists believe we are already seeing evidence for an upswing in the numbers of the most powerful storms; others are less convinced.
More Clouds: Clouds 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 at different locations have different effects on climate. Some shade the Earth, cooling climate. Others enhance the greenhouse effect with their heat-trapping water vapor and droplets. 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.
Changes to Life 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. There will also be changes to the lengths of growing seasons, geographical ranges of plants, and frost dates.
Models of the global carbon cycle suggest that the Earth system will be able to absorb less CO2 out of the atmosphere as the climate warms, worsening the warming problem.
Even if greenhouse gas emissions were halted immediately, we are committed to a certain amount of global warming (an estimated 0.5° C) because of the amounts of these gases already present in the atmosphere. This is called “climate commitment”. Past greenhouse gas emissions have committed us to certain amounts of warming because these gases leave the atmosphere very, very slowly.
This climate commitment means that we are committed to certain impacts of warming such as ocean warming and sea ice melt. Climate commitment is a minimum based on what has already happened. It can be increased by continued emissions of greenhouse gases.
Possible Abrupt Changes & Tipping Points
Some changes to climate are gradual and predictable, while others are more sudden and difficult to foresee. The latter are often referred to as "tipping points". A tipping point is a large, abrupt change that cannot readily be stopped at the last minute even by employing drastic measures.
Possible tipping points include:
None of these tipping points are considered very likely to occur in the short run; but the consequences of any of them are so severe, and the fact that we cannot retreat from them once they've been set in motion so problematic, that we must keep them in mind when evaluating the overall risks associated with climate change.