Present Climate: How Do We Know Earth is Warming Now?
The Greenhouse Effect
Without the so-called greenhouse gases, including carbon dioxide, methane, nitrous oxide, and water vapor, Earth would be too cold to inhabit. These gases in Earth's atmosphere absorb and emit heat energy, creating the greenhouse effect that keeps our planet's temperature livable.
Too Much of a Good Thing
Since the industrial revolution, people have burned vast amounts of coal, petroleum, and other fossil fuels to create heat and power. This releases carbon dioxide, the most plentiful greenhouse gas, into the atmosphere. The result: more heat is trapped in Earth's atmosphere instead of radiating out into space.
Because carbon dioxide lasts more than a century in the atmosphere, it is well mixed around the globe. Measurements collected atop Hawaii's Mauna Loa since 1958 show a steady rise in global carbon dioxide concentrations. These have increased by 35% since preindustrial times, according to the U.S. Department of Energy and the World Meteorological Organization. Other, less prevalent greenhouse gases have increased at different rates. Methane, for example, has virtually leveled off since 1999 at 155% above its preindustrial level.
For more than 100 years, Earth's surface temperature has been monitored by a global network of land-based weather stations. These reports are supplemented by sea-surface and air temperature readings taken at points across the oceans that cover 70% of the globe. The ups and downs of air temperature are modulated by the sea, so the uppermost ocean serves as a good index of the average air temperature just above it.
Together, these data show that Earth's surface air temperature has risen about 1°F (0.7°C) since the late 1800s. This warming of the average temperature around the globe has been especially sharp since the 1970s. Global models at NCAR have simulated 20th century climate and found three main factors at work:
Solar activity contributed to a warming trend in global average temperature from the 1910s through 1930s.
As industrial activity increased following World War II, sun-blocking sulfates and other aerosol emissions helped lead to a slight global cooling from the 1940s to 1970s.
Since 1980, the rise in greenhouse gas emissions from human activity has overwhelmed the aerosol effect to produce overall global warming.
Model results confirm the importance of human-produced emissions in the temperature trends of recent decades. This graphic depicts global average temperature since 1890 as reproduced by the NCAR/DOE Parallel Climate Model. The blue line summarizes simulations performed using only natural influences on climate (volcanoes and solar variations). The red line, from a set of simulations that includes sulfates and greenhouse gases, is much closer to the observed record (black line). The blue and red shading shows the range of results (the model uncertainty) for each group of simulations. (Illustration courtesy Gerald Meehl, NCAR.)
Some urban areas have also warmed due to the heat-island effect, where buildings and pavement retain more heat than undeveloped areas and cause more runoff and thus drier conditions as well. NCAR scientists and their colleagues have worked carefully to remove urban heat-island effects and other potential biases from the global record. Even after these adjustments, the rise in global temperature remains clear.
There are other signs of a warming planet. Glaciers are retreating, especially atop lower-latitude mountains. In the Arctic the thickness and extent of the sea ice have decreased dramatically over the last 50 years. Snowfall over much of Antarctica is increasing, a paradoxical sign of warming temperatures in this frozen land. The annual cycle of plants and migrating animals shows a lengthening of the warm season over much of the Northern Hemisphere.
Since the late 1970s, satellites have measured the temperature in a broad layer of the troposphere several miles above Earth; they indicate a smaller temperature rise at these heights than at ground level. The difference was accounted for in 2004, when researchers at the University of Washington and colleagues factored in satellite readings of the cooler temperatures of the stratospheric layer above the troposphere. Scientists expected this cooling of the stratosphere as carbon dioxide increased and ozone depleted. Once the cooling effect of stratospheric air was accounted for, the satellite readings for the troposphere matched or exceeded the surface temperature trend.
Studies of the past, present, and future climate benefit continually from improvements in data gathering, computer modeling, and analysis. For example, recent research at NCAR re-examines the role of decades-long cycles of solar variation in explaining the observed warming in the first half of the 20th century.
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