Today marks the 150thanniversary of the creation of the U.S. National Academy of Sciences. It may seem impossible in today’s gridlocked environment, but in 1863, President Lincoln proposed it, and Congress passed the law creating the National Academy (NAS) in a matter of days. Its mission was to advise our government about matters of science and engineering that were beyond the training of politicians. And the Industrial Revolution was expanding exponentially the number and complexity of such issues.
|NAS climate guidance: Free download|
- Evidence for Human-Caused Climate Change
- How do we know that Earth has warmed? Widespread measurements of temperature around the world began around 1880. These data have steadily improved and, today, temperatures are recorded in many thousands of locations on the land, over the oceans, and from satellites, ocean sensors and weather balloons. These analyses all show that Earth’s average surface temperature has increased by more than 1.4°F (0.8°C) over the past 100 years, with much of this increase taking place over the past 35 years. 1.4°F is like the difference between the average temperature in Washington, DC and Charleston, SC.
- How do we know that greenhouse gases lead to warming? In 1824, French physicist Joseph Fourier was the first to discover the greenhouse effect. In the 1850s, Irish-born physicist John Tyndall demonstrated the greenhouse effect as scientific fact. In 1896, Swedish scientist Svante Arrhenius calculated the warming power of excess CO2, predicting that if human activities increased CO2 levels, climate warming would result. Much research has built on these classics, but they are no longer in doubt.
- How do we know that humans are causing greenhouse gases to increase? CO2 is produced and consumed in many natural processes that are part of the carbon cycle. However, once humans began digging up long-buried forms of carbon such as coal and oil and burning them for energy, additional CO2 began to be released into the atmosphere much more rapidly than in the natural carbon cycle. Other human activities, such as cement production and deforestation also add CO2 to the atmosphere. Today, atmospheric CO2 concentrations exceed 390 parts per million—nearly 40% higher than preindustrial levels, and, according to ice core data, higher than at any point in the past 800,000 years.
- How much are human activities heating Earth? There are several well-known human impacts on climate: greenhouse gas emissions are the most familiar; emissions from burning fuels also produce small atmospheric particles called aerosols, which have a slight cooling effect; and land-use changes (farming, urban development and deforestation) also have – surprisingly – a slight overall cooling effect. But when these human impacts are taken together, the net climate impact is pushing the earth toward warming.
|Ice core data: CO2 and temperature move together|
- How do we know the current warming trend isn’t caused by the Sun? Since 1979, we have measured the Sun’s energy from satellites. These records show that the Sun’s output has not shown a net increase during the past 30 years and thus cannot be responsible for the warming during that period. And since the 1950s, weather balloons have measured temperatures in both the lower and upper atmosphere. More intense heat from the Sun would warm all layers; but in fact, the upper layers have cooled, and the lower layers have warmed. Satellite and weather balloon measurements have been closely scrutinized, and both show a warming trend in the lower layer of the atmosphere and a cooling trend in the upper layer. The Sun is not the cause of today’s global warming.
- How do we know the current warming trend isn’t caused by natural cycles? Many natural factors affect the Earth’s climate, including volcanic activity, El Nino and La Nina cycles, and small changes in our orbit around the Sun. Scientists have modeled the impact of these natural impacts on climate over the last 50 years, and have concluded that their combined effects have been largely neutral to slightly cooling. There is a more than 90% chance that most of the observed global warming trend over the past 50 to 60 years can be attributed to emissions from the burning of fossil fuels and other human activities.
- What other climate changes and impacts have been observed? Global warming is not uniform all over the globe, and its effects vary from place to place. Major observed impacts include:
- Warming, Climate Changes, and Impacts in the 21st Century and Beyond
- How will temperatures be affected? By the end of the century, the center of the United States is expected to experience 60 to 90 additional days per year in which the heat index is more than 100°F. The ratio of new record high temperatures to record low temperatures currently stands at 2 to 1. But that ratio is projected to increase to 20 to 1 by mid-century and 50 to 1 by the end of the century.
- How is precipitation expected to change? Globally, dry areas are expected to get even drier and wet areas even wetter. Some notable details:
- How will sea ice and snow be affected? In the Arctic, sea ice will decline 25% for each 1°C (1.8°F) in global warming this century. In the Antarctic, sea ice is expanding due to the stratospheric “ozone hole” which developed because of the use of ozone-depleting chemicals; this effect is expected to wane as ozone returns to normal levels by later this century. In many areas of the globe, snow cover is expected to diminish, with snowpack building later in the cold season and melting earlier in the spring. Each 1°C (1.8°F) of local warming may lead to an average 20% reduction in local snowpack in the western United States; reduced snowpack will restrict summer drinking water supply and hydropower production.
- How will coastlines be affected? Sea-level rise is projected to continue for centuries in response to human- caused increases in greenhouse gases, with an estimated 20-39 inches of mean sea-level rise by 2100, and more beyond. However, there is evidence that sea level rise could be much greater, due to unexpectedly rapid melting from glaciers and ice sheets. If sea levels rise by 39 inches this century, many parts of the U.S. coastline will be impacted (see map below). In any event, global “hotspots,” including the Mississippi, Ganges and Mekong River deltas will be seriously affected.
- How will ecosystems be affected? Species are adapted to specific climatic conditions in their ecosystems. As the climate changes, many species will be forced to migrate. But this threatens many species with limited mobility. Special stress is being placed on cold-adapted species on mountain tops and at high latitudes, which cannot move higher or further pole-ward. Shifts in the timing of the seasons and life-cycle events such as blooming, breeding, and hatching are causing mismatches between species, disrupting patterns of feeding, pollination, and other key aspects of food webs.
|Large losses to American cities from 39” sea rise (pink shading)|
- How will agriculture and food production be affected? Warmer weather and increased CO2 do not necessarily mean a decline in food production. However, temperature increases above 1°C (1.8°F) will reduce yields in almost all of the world’s staple foods, a result of water stress and temperature peaks. Local results will also vary, with a projected 40% yield decrease in a broad cross section of California crops by 2050. Growers in prosperous areas may be able to adapt to some climate threats. However, adaptation may be less effective where local warming exceeds 2°C (3.6°F), and will be limited in many poorer countries and the tropics, where crop yields are restricted principally by moisture.
- U.S. National Academy of Sciences, “Climate Change: Evidence, Impacts and Choices”
- U.N. Intergovernmental Panel on Climate Change, “Fourth Assessment Report: Summary for Policymakers”
- Woods Hole research Center, “Understanding Climate Change: A Primer”