New insights: global warming drivers in the 20th century and beyond
Laura Nielsen for Frontier Scientists
Researchers have combed through the last 2,000 years of climate records. Their assessment affirms that a persistent long-term cooling trend concluded in the late 19th century, reversed by global warming. The study was performed by members of the "2K Network" of the International Geosphere Biosphere Program (IGBP) Past Global Changes (PAGES) project, supported by both the U.S. National Science Foundation and the Swiss National Science Foundation.
The international effort utilized 'proxy data' to discern temperatures during the last two thousand years. Ice cores and lake sediment cores were drilled and examined - those cores can hold layers of pollen, volcanic ash, microscopic plankton with unique chemical fingerprints, or bubbles of air trapped in ice which hold tiny samples of past atmosphere. Each of those layers indicate facts about climate information. Tree rings tell as story of ancient air temperatures, just as the banded skeletons of dome corals log sea temperatures.
The cooling trend documented had complex drivers, including fluctuations in the sun's activity and heightened volcanic activity. 1816, sometimes called 'The Year Without a Summer', paints a picture of both. The sun was undergoing a dip in solar activity known as the Dalton Minimum. More importantly, a series of volcanic eruptions between 1812 and 1815 spewed ash and other atmospheric particulates into the sky, causing less sunlight to successfully pierce the atmosphere. The 1815 eruption was the famous eruption of Mount Tambora in Sumbawa, Indonesia, which met a Volcanic Explosivity Index ranking of 7.
While the study did not specifically focus on anthropogenic (human-caused) temperature changes, it is abundantly clear that warming during the 20th century cannot be explained away without factoring in humans. According to Paul E. Filmer, director of programs in the National Science Foundation's Geosciences Directorate: "The natural forces driving the cooling are still present today, but since the nineteenth century an additional, stronger, warming driver has been added: human activity. We cannot match the temperature records since then without factoring in this new driver." The research is vital because understanding earth's past climate can help us gauge and predict future climate.
As rising atmospheric concentrations of greenhouse gasses and other factors continue to heat up our world, adding more moisture to the atmosphere, we can expect to face more extreme precipitation events. Climate models run on supercomputers forecast intense precipitation events like hurricanes, floods and droughts, can be expected to be both more frequent and more severe. The National Oceanic and Atmospheric Association also forecasts that climate warming will directly reduce global labor capacity during hot months because of heat stress. NOAA's "Climate Adaptation Strategy provides a roadmap of key steps needed over the next five years to reduce the current and expected impacts of climate change on our natural resources, which include: changing species distributions and migration patterns, the spread of wildlife diseases and invasive species, the inundation of coastal habitats with rising sea levels, changing productivity of our coastal oceans, and changes in freshwater availability."
We do need to adapt. The most ordinary practices, like conducting controlled burns for agriculture, irrigating fields, creating drainage swamps, and even building dams, can have unforeseen meteorological consequences.
Burn scars -dark spots on the land- promote formation of thunderstorms. The sun heats the dark spot, causing hot air to rise rapidly. Cooler air rushes into the vacated space, meaning heavy clouds can form and cause heavy downpours just downwind of the scar. These trends replace more evenly spread moisture and rainfall. Similarly large dams alter rainfall patterns and promote thunderstorms, since air over large reservoirs can soak up evaporated water in one concentrated location, causing heavy precipitation in the dam's vicinity and altering normal precipitation patterns.
Urbanization, shipping, and agricultural practices which pump black carbon (soot) and dust into the atmosphere also have unforeseen consequences. Just like volcanic ash in the atmosphere blocks sunlight, dust can have a cooling nature; however, once it lands on snow or ice dust instead promotes warming. Clean bright white ice has a high albedo, or solar reflectiveness, and sends the sun's rays back into space like a giant mirror. Dark ice or snow dirtied by soot and dust, on the other hand, absorbs the sun's heat and causes melting. A melting polar ice cap means more dark ocean exposed to sunlight... yet more energy absorbed from the sun. Thinner sea ice which replaces lost multi-year ice is vulnerable to melting and adding to clouds and water vapor. Thin low-lying clouds let sunlight through, but then trap it near the surface of the earth. It is clouds like that which drove the alarming Greenland ice melt last summer.
Of course greenhouse gasses like Carbon dioxide (CO2) and Methane (CH4) also promote atmospheric heat. We introduce these gasses to the atmosphere by burning fossil fuels, deforestation, and keeping huge populations of ruminant livestock like cattle. Once there, rising temperatures open up new sources. Heightened temperatures in the Arctic are causing permafrost to thaw, meaning ancient frozen plant matter has begun to decay and add yet more Carbon dioxide and Methane to the atmosphere. As temperatures and CO2 concentrations rise, each amplifies the other in a positive feedback loop. “Antarctic ice cores show us that the concentration of CO2 was stable over the last millennium until the early 19th century. It then started to rise, and its concentration is now nearly 40% higher than it was before the industrial revolution… The fastest large natural increase [of CO2] measured in older ice cores is around 20ppmv (parts per million by volume) in 1000 years (a rate seen during Earth’s emergence from the last ice age around 12,000 years ago). CO2 concentration increased by the same amount, 20ppmv, in the last 11 years!” according to the Natural Environment Research Council.
Aggressive plans to reduce heat-trapping carbon dioxide emissions, and more careful examinations at how many of our activities drive environmental change, are necessary if we are to mitigate climate change. Studies like the "2K Network" data synthesis help us understand the climate of the past, and drive home the fact that we live in a complex system. Many factors play a role in determining Earth's future climate.
* 'Analysis of 2,000 Years of Climate Records Finds Global Cooling Trend Ended in the 19th Century,' National Science Foundation news http://www.nsf.gov/news/news_summ.jsp?cntn_id=127658&WT.mc_id=USNSF_51&WT.mc_ev=click
** 'National strategy will help safeguard fish, wildlife and plants in a changing climate,' National Oceanic and Atmospheric Association news http://www.noaanews.noaa.gov/stories2013/20130326_climate_adaptation_strategy.html
*** 'Science briefing - Ice cores and climate change,' British Antarctic Survey http://www.antarctica.ac.uk/press/journalists/resources/science/icecorebriefing.php
'Probable Maximum Precipitation and Climate Change,' Geophysical Research Letters http://onlinelibrary.wiley.com/doi/10.1002/grl.50334/full
'New NOAA study estimates future loss of labor capacity as climate warms,' National Oceanic and Atmospheric Association news http://www.noaanews.noaa.gov/stories2013/20130225_laborandclimate.html
'Have Large Dams Altered Extreme Precipitation Patterns?," American Geophysical Union http://pielkeclimatesci.files.wordpress.com/2009/12/r-349.pdf
'Thin, Low Arctic Clouds Played an Important Role in Widespread 2012 Greenland Ice Sheet Melt,' National Science Foundation news http://www.nsf.gov/news/news_summ.jsp?cntn_id=127438