While natural forces have influenced Earth’s climate (and always will), human-induced changes in atmospheric greenhouse gas levels are playing an increasingly dominant role.
The significant increase in average global temperatures over the past half-century can be attributed to human activities with a certainty of more than 90 percent.
Temperature rises have already affected various natural systems in many regions.
Continued greenhouse gas emissions at or above current rates would cause further warming and induce climate changes during the 21st century that would very likely be larger than those observed during the 20th century.
Below I quote liberally from the Woods Hole Oceanographic Institute, but there is much more at the site. These are just the key take home points (the fact there are a lot of “key take home points” is a sign in and of itself that we have a serious problem on our hands.)
Temperature and CO2 levels on the increase:
- CO2 levels have risen 35 percent since the industrial revolution began in the mid-18th century and are likely at their highest levels in the past 20 million years. The main source is the burning of fossil fuels, such as oil, natural gas, and coal.
- Earth’s average temperature has been increasing over the past century (albeit not uniformly), with warming accelerating over the past 50 years. Of the hottest 12 years since temperatures began to be measured in the 1850s, 11 have occurred in the past 12 years.
- No known natural forcing can account for the recent severe warming.
Changes in Precipitation patterns:
- Changes in precipitation amounts have been detected over large portions of the world.
- Warmer air temperatures induce more evaporation, drying some areas of the globe.
- Warmer air also holds more water vapor, leading to heavy rains, when this higher water-content atmosphere drops its moisture.
Warming Oceans, Changes in Salinity:
- Increased evaporation leaves some areas of the ocean more salty, while increased rainfall adds fresh water to other areas
- Oceans in the mid- and high latitudes show evidence of freshening, while those in tropical regions have increased in salinity.
- Global mean sea surface temperature increased 0.9°F in the 20th century, and the IPCC stated that “global ocean heat content has increased significantly since the late 1950s.”
Increasing extremes in weather:
- Since 1950, cold days and nights and frost days have become less frequent, while hot days and nights and heat waves have become more frequent.
- Heavy precipitation events have increased and droughts have become more intense, particularly in the tropics and subtropics, because of higher temperatures and changes in precipitation patterns.
- The combination of increasing air temperatures and sea surface temperatures can increase the energy of tropical storms.
Loss of Ice and Snow:
- Glaciers are retreating and ice and snow cover are disappearing in many regions around the world.
- Melting ice exposes land or water, both of which reflect less solar radiation than ice. That reinforces rising temperatures, which melt more ice. Once such loops begin, their endpoint is hard to predict.
- Increased melting of the vast Greenland Ice Sheet may make it vulnerable to sudden, catastrophic breakup.
Arctic in Danger:
- Glaciers are melting, permafrost is thawing, land is subsiding, the snow season has shortened, and sea ice is thinning and shrinking.
- Little to no sea ice is expected in the Arctic’s summers by 2100.
Rising Sea Levels:
- As ocean temperatures increase, water expands, causing sea levels to rise. Once sea level begins to rise because of thermal expansion, it will continue to do so for centuries regardless of mitigative actions.
- Sea levels have risen 7 inches over the 20th century, and nearly 1.5 inches between 1993 and 2003.
Ocean Acidity Change:
- Increased atmospheric CO2 is absorbed in the ocean where it combines with water to form carbonic acid.
- Forecasts project the increase in acidity over the coming century to be three times as great as the increase over the past 250 years.
- Higher acidity could have a major impact on ocean life by preventing the formation of shells and skeletons of abundant and important zooplankton. Coral reefs are particularly vulnerable.
What does this abrupt climate change mean for us?
- For many years it was believed that climate changes have been gradual—that the Earth gradually cycles between glacial periods and warm interglacial periods. We now know this is not always the case.
- Such abrupt climate changes could make future adaptation extremely difficult, even for the most developed countries.
A summery of the Human Toll:
• Billions of people will be exposed to stresses on their water supplies.
• Climate change will exacerbate water stress in some regions and alleviate it in others.
• Developing nations with little capacity to manage water resources will be hardest hit.
• Areas that depend on water supplies stored in glaciers and snow cover (more than 16% of world population) face scarcities as glaciers continue to melt and eventually disappear.
• Increased temperatures, heat waves, precipitation changes, drought, and pests will harm agricultural production over much of the globe, though some regions will benefit.
• Coastal populations will be exposed to more flooding, erosion, and inundation from rising sea levels and more intense storms, especially in low-lying areas and on small islands.
• Rising temperatures and heat waves will increase the number of heat-related deaths (outweighing a decrease in deaths from cold exposure).
• Higher ozone levels will increase the frequency of cardiorespiratory disease.
• Climate changes will help spread vector-borne and pathogenic diseases such as malaria, dengue fever, and cholera.
• Increased flooding will harm human health directly, and also indirectly—by facilitating the spread of disease and damaging health infrastructure.
• About 20% to 30% of plant and animal species are at increased risk of extinction.
• Progressive acidification of the oceans will have negative impacts on marine organisms critical to the ocean food web.
• Widespread mortality of biodiverse coral reefs is expected.
• Decreased rainfall in some regions will increase the risk of wildfires.
• Damages from climate change are likely to be significant and increase over time. Global mean losses could be 1% to 5% of gross domestic product for a likely 4°C warming over the next century.
I cut and pasted the highlights, but there is much more here.