Questions and Answers on the Science of Climate Change

Q: What does “global warming” really mean?

A: People often refer to “global warming” to describe increasing temperatures related to human activities such as fossil-fuel burning and deforestation. These activities add greenhouse gases to the atmosphere, which in turn trap more heat near the Earth’s surface, like a blanket. Global warming can trigger many other changes, including the melting of glaciers and polar ice sheets, rising sea levels, acidification of the oceans, shifts in rainfall patterns, more extreme rain, and more floods, droughts, and wildfires. Global warming also may affect the distribution of diseases such as malaria and dengue fever; the timing of plant flowering and animal breeding; the health of coral reefs; and farm productivity, among others.

Q: Is global climate change real?

A: Yes. Today, the vast majority of scientists agree that the Earth is warming and will continue to warm as a result of human-produced greenhouse gas emissions.

Between 1990 and 2100, if emissions continue to increase, temperatures are expected to rise between 2 and 11.5 degrees Fahrenheit (1.1 to 6.4 degrees Celsius). Since 1880, average global temperatures have increased by 1.5 degrees Fahrenheit (0.8 Celsius), according to a January 2010 report by the U.S. National Aeronautics and Space Administration (NASA). Over the past three decades, average global temperatures have climbed about 0.36 degrees Fahrenheit (0.2 degrees Celsius) per decade.

Scientists at the Alaska Climate Research Center have found that average temperatures there have increased by 3.5 degrees Fahrenheit (1.9 degrees Celsius) over the last five decades and by 6.3 degrees Fahrenheit (3.5 degrees Celsius) over the winters.

The 1990s had been listed as the warmest decade ever recorded—until January 2010, when scientists reported that the global average temperatures in the first decade of the 21st century were even higher.

Q: But we have had an unusual amount of snow where I live. How could the Earth be getting warmer?

A: Day-to-day or short-term weather fluctuations should not be confused with much longer-term climate patterns, nor should weather in one area be considered representative of global conditions. But it is possible that climate change may affect the occurrences of extreme or severe events or marked changes in rainfall patterns.

Q: Are humans causing global climate change?

A: Yes. Some 7.2 billion tons of carbon dioxide are being released into the atmosphere every year as a result of human activities. Cars, power plants, factories, and homes release about 5.5 billion tons of carbon dioxide every year as fossil fuels such as coal, oil, and natural gas are burned to produce energy. Another 1.6 billion tons of carbon dioxide are released by deforestation, especially in the tropics. Some of this carbon dioxide is absorbed by the ocean, plants, and other natural “sinks.” But about 3.2 billion tons of carbon dioxide remains in our atmosphere every year.

Q: Do natural climate fluctuations cause warmer weather?

A: Weather is what we experience from day to day or even year to year; climate is how weather changes over decades, or centuries, or longer. Although the climate may change from age to age, the warming trend of the last century is unprecedented in the historical record and clearly results from human activities. By studying ancient Antarctic ice cores, scientists have mapped temperatures and carbon dioxide levels over the past 800,000 years. These records clearly show that greenhouse gases have amplified natural warming cycles.

Since the Industrial Revolution, carbon dioxide levels have jumped from 280 parts per million (ppm), a level characteristic of the interglacial periods, to 389 ppm at the end of 2009. Temperatures on Earth also began to rise. The first decade of the 21st century was the warmest ever recorded.

Q: What climate-related changes can we expect in coming decades?

A: As 2009 ended, the concentration of atmospheric carbon dioxide—resulting mostly from the human activities of deforestation and burning fossil fuels—stood at 389 ppm. Carbon dioxide (CO2) levels are rising at a rate of about 2 ppm per year, and therefore could hit 400 ppm with this decade. Some scientists have said that average temperatures could jump by as much as 4 degrees Fahrenheit (about 2.2 degrees Celsius) if the atmospheric CO2 reaches 450 ppm. We may face even more dangerous impacts at 550 ppm, and above that level, potentially devastating events. U.S. crop productivity would be affected, while and communities in some regions might suffer increased fatalities because of intensely hot summers.

Already, it is clear that our children and grandchildren will inherit a dramatically different world. In North America and Europe, researchers have documented that signs of spring such as animal breeding and the blooming of flowers are occurring, on average, 2.3 days earlier over the course of each decade for the past half-century. All over the world, glaciers and ice sheets are shrinking dramatically, and research shows the melting is caused by a warming climate.

Average sea level worldwide may rise by 7.2 to 23.6 inches (18 to 59 cm) by the end of the century. This estimate may be conservative because it assumes that the flow of melted water from Greenland and Antarctica will remain constant with rates from 1993-2003.

Even now, Arctic settlements such as the village of Shishmaref, Alaska, are preparing to relocate because of rising waters and coastal erosion. The government of the Maldives, an archipelago in the Indian Ocean, has explored relocation of nearly 400,000 residents because rising seas threaten to submerge the low-lying islands.

Q: How else can global climate change affect us?

A: Global climate change affects the productivity of farms, forests, and fisheries; the livability of cities in summer; and damages from storms, wildfires, floods, and droughts. Recent research has linked a sharp increase in the number of wildfires in western U.S. forests since the mid-1980s with warming temperatures and earlier snow melts in the mountains. Other studies have raised the possibility that the intensity of tropical storms— hurricanes and typhoons—could increase in all ocean basins.

Climate change affects the distribution and abundance of every species on Earth, including creatures that we value and depend upon, but also pests that carry diseases such as malaria. The World Health Organization has estimated that, in the year 2000, climate change claimed some 150,000 lives and sickened many others, especially elderly people and children. Continuing climate change is likely to kill or sicken many more.

Q: Why are scientists so concerned about the Greenland ice sheet?

A: The Greenland ice sheet is rapidly thinning, and ice loss seems to be happening much more quickly than previously assumed. In fact, satellite measurements from 1996 to 2005 revealed a doubling of the amount of ice being dumped by Greenland’s glaciers into the Atlantic Ocean. It’s important to note that scientists aren’t certain yet how fast the ice is melting, and they also don’t know how much atmospheric carbon dioxide might cause enough warming to trigger a sudden disintegration of the Greenland ice sheet. But, a number of respected, cautious scientists believe there is a “trigger point,” which may lie somewhere between 400 ppm and 700 ppm atmospheric carbon dioxide. At the end of 2009, the atmospheric CO2 level was at 389 ppm. The Greenland ice sheet contains enough ice to raise average sea level by around 6.5 meters (about 21 feet). Although the ice sheet is not thought to be in danger of complete collapse, and sea-level rise caused by its melting and decay probably would happen only over hundreds or thousands of years, it is projected to be enough to swamp coastal communities all over the planet.

Q: Is global climate change related to the “ozone hole” problem?

A: The “ozone hole” over Antarctica is different from global climate change, although both problems are caused by pollution. High above the Earth, the ozone layer protects us from the Sun’s ultra-violet rays. But, chlorine in chemicals such as chlorofluorocarbons, or CFCs, can interact with ozone molecules on ice particles in a way that destroys the ozone layer. In the 1990s, the United States played a leading role in addressing the ozone problem, by taking decisive action to reduce the production and use of ozone-destroying chemicals. But climate change, while warming the troposphere, cools the stratosphere where ozone resides. This cooling may slightly increase the presence of ice, affecting the recovery of the hole. And CFC’s are also greenhouse gases.

Q: If climate change is real, why do some physical signs seem contradictory?

A: Some climate-change skeptics have noted that the middle of the Greenland ice sheet and the center of Antarctica are growing thicker, and therefore must be getting larger, rather than melting. In fact, a thickening around the middle of Greenland and Antarctica has been caused by increased precipitation linked to global warming. And studies have found an overall reduction in both the Greenland ice sheet and Antarctica over the past several years. Global climate change disrupts the entire climate “envelope” on our planet and can cause extreme fluctuations in regional weather patterns, from snowfall to droughts. Global climate change is thus far more complex than global warming and melting ice.

Q: How can we fix the problem without ruining our economy?

A: The tools exist today to keep greenhouse-gas levels from rising any higher. AAAS and others have suggested approaching the problem from three directions—by combining efforts to reduce energy consumption and greenhouse-gas emissions with strategies for adapting to climate change as well as technologies designed to mitigate impacts.

Human adaptation can include, for example, changing settlement patterns, or farming and flood-management practices.

Improving the energy efficiency of cars, trucks, planes, buildings, appliances, and manufacturing processes, combined with next-generation energy solutions and innovative new technologies for capturing or “sequestering” carbon may also hold the key to our climate future. Increasingly, leading international corporations are setting the pace in embracing cutting-edge research to address global climate change.

Q: What can I do?

A: Small steps can have big impacts. Plant trees, which absorb carbon dioxide. Consider more energy-efficient technologies, such as a hybrid car, or simply use less heat and air-conditioning. Become politically active, and vote for leaders who promise to work toward climate-change solutions. Calculate your own “carbon footprint,” then try to reduce it.

AAAS has more resources on climate change at www.aaas.org/climate.