Irreversibility?

Ongoing CO2 emissions contribute to existing atmospheric conditions, ensuring that by the end of the 21st century, carbon dioxide will be the main determinant of global warming. The deleterious effects of current GHG emissions will last for centuries. Even if all anthropogenic CO2 emissions were to end right now, climate change will continue. According to one scenario, about 15% to 40% of CO2 will stay in the atmosphere for another 1,000 years.

“Drivers of Climate Change”

The Earth’s energy source is the sun. Over time, the energy it emits to Earth and the solar radiation the planet gives off tend to be in balance. Changes in the “Earth’s radiation budget” – whether due to human activity or nature – will affect the planet’s air, water, “biosphere” and cryosphere. This “anthropogenic” – or naturally occurring “radiative forcing” (RF) – influences climate change. Higher GHG concentrations interact with the natural effects of clouds and surface areas to bring unanticipated changes to the Earth’s temperatures. Now, “total RF is positive,” which means the Earth’s surface is warming. Since 1750, RF has increased, rising faster since 1970. From 2005, anthropogenic RF grew 43%, largely as a result of more GHG concentrations.

Human impact on the climate is irrefutable, based on observed higher levels of GHGs, a positive RF, greater temperatures and better knowledge of the Earth’s climate system. As scientists study air and sea warming, water cycle changes, ice surface shrinkage, sea level rise and more extreme weather patterns, they find it extremely likely that humans are predominantly responsible for climate change since the mid-20th century.

Anthropogenic impacts are the extremely likely cause of “more than half of the observed increase” in global temperatures since then. Greenhouse gases are the culprits. In addition, those human-led impacts are now very likely to be accountable for extremes in daily temperatures, including heat spells, sea level increases, and glacier and Arctic sea ice losses.

Future Temperatures

By 2100, Earth’s surface temperature is likely to increase by 1.5ºC relative to temperatures from 1850 to 1900. Differences will occur across the world and through time. Scientists forecast with medium certainty that temperatures from 2016 through 2035 will be 0.3ºC to 0.7ºC higher than those from 1986 to 2005. Tropical areas can expect greater seasonal increases. Experts are virtually certain that incidences of extremely high temperatures will outnumber those of extremely low temperatures. More and longer heat waves are likely, though there may be extremes in cold winter temperatures from time to time.

Future “Water Cycles”

Precipitation levels will vary over the 21st century, with greater variability among seasons and more differences showing up between damp and arid areas. One scenario calls for higher average precipitation over the upper latitudes and the equator in the Pacific Ocean. By 2100, typically dry regions will see less rain and rainy areas will see more. As temperatures rise, incidents of “extreme precipitation” in the tropics and mid-latitudes will be more common and severe. Monsoons will cover ever-larger regions. Wind strength will likely diminish, but monsoons will dump much more rain than normal. In many areas the monsoon season is likely to begin sooner and end later. Experts are highly confident that the El Niño-Southern Oscillation pattern will dominate in the Pacific in the 21st century; increased moisture will spread its effects globally.

Future Air Quality

Estimates of air quality in the 21st century depend more on levels of GHG emissions than on climate change impacts. Warmer temperatures are likely to interact with pollutants to raise ozone levels. Ocean temperatures will continue to rise in this century, warming water from the surface to the depths, and disturbing “ocean circulation.” Oceans in tropical zones and in Northern Hemisphere subtropical areas will be warmest. Projections for ocean temperatures from the surface to 100 meters call for increases of 0.6ºC to 2.0ºC by century’s end. Depths of up to 1,000 meters could see temperature increases of 0.3ºC to 0.6ºC.

The Future Cryosphere

Sea ice, snow cover and glaciers will continue to diminish. Researchers now call for “year-round reductions in Arctic sea ice extent” by 2100. One scenario calls it likely that by 2050 the Arctic Ocean will be “nearly ice free” by the month of September each year. Most glaciers, except for those on Antarctica’s edges, will drop in volume by anywhere from 15% to 85%, depending on the analysts’ model and scenario. Spring snow cover in the Northern Hemisphere will decline by 7% to 25%. Surface permafrost in the high northern latitudes will disappear at a rate of 37% to 81% at the end of the 21st century.

Future Sea Levels and Carbon Cycles

No matter which scenario scientists choose, each projection forecasts that average sea levels will very likely increase throughout this century to levels above those from 1971 to 2010. Scientists’ increased certainty regarding their projections is due to greater knowledge of how various environmental aspects of climate change affect sea levels. Depending on region and scenario increases range from an average of 0.26m to 0.98m by 2081 through 2100, compared to levels measured from 1986 to 2005. “Thermal expansion” will make up 30% to 55% of the rise, while melting glaciers will contribute 15% to 35%. Seas will be higher in different parts of the world. By 2100, 95% of oceans are highly likely to rise. This will affect 70% of the globe’s coastlines. The environmental effects of climate change will become worse when they interact with anthropogenic GHGs. Greater amounts of CO2 will only increase the ocean’s acidity.

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