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Frequently Asked Questions

What is the greenhouse effect?

The greenhouse effect helps to maintain and regulate the temperature of our planet. Ever since the industrial revolution began, factories, power plants and eventually cars burnt fossil fuels such as oil and coal, releasing huge amounts of gases such as carbon-dioxide, methane and others into the atmosphere. These greenhouse gases trap heat near the Earth in a naturally occurring process known as the ‘Greenhouse Effect’. The greenhouse effect begins with the sun, and the energy it radiates into the Earth. The Earth and the atmosphere absorb some of this energy while the rest is radiated back into space. Naturally occurring gases in the atmosphere trap some of this energy and reflect it back warming the Earth. But over the last few years, enhancement of the greenhouse effect through more greenhouses gases is causing the temperature of the Earth to rise unusually fast.

What is causing greenhouse gases to increase?

Human activity is causing greenhouse gases to increase in the atmosphere, mainly carbon dioxide. Carbon dioxide is released into the atmosphere through the burning of fossil fuels such as coal, oil and natural gas. Before the industrial revolution, the level of carbon dioxide in the atmosphere was 280 parts per million by volume and the current levels are greater than 380 parts per million by volume. If the level if carbon dioxide continues to rise at the present rate, then carbon dioxide levels are estimated to rise to anywhere between 490 to 1260 parts per million by volume.

Are sea levels rising?

 Yes. Global mean sea level has been rising at an average rate of 1.7 mm/year (plus or minus 0.5mm) over the past 100 years, which is significantly larger than the rate averaged over the last several thousand years. In India, the Himalayas are retreating at three times the normal rate, from 19 meters in 1971 to 34 meters today. Arctic sea ice is also sinking, which is causing water levels to rise at an alarming rate.

What are its likely consequences in the future?

Global temperatures are anticipated to rise anywhere between 1.5°C to 3.0°C in the next 50 years. Rising sea levels could flood coastal areas around the world. Weather patterns could change, making hurricanes more frequent. Severe droughts would increase in warm areas and species unable to adapt to the changing environment could face extinction. Also, there would be devastating effects on population growth, economic growth and the entire civilization as a whole.

So, what should we choose? ‘Economic Growth’ or the ‘World’?

Firstly, if you did not have a world, there would be no economic growth, and secondly, a well designed programme along with small and effective initiatives taken by people by making some conscious amendments to the way in which they lead their daily lives will ensure us to combat this problem without having to hamper economic growth.

Yet, there are people who do not believe that global warming exists.

Well, there are people who yet believe that there was no Nazi Holocaust against the Jews.

Global warming or climate change: Which is it?

Two 19th-century scientists are associated with the discovery that increasing carbon dioxide in the atmosphere warms the entire planet: French researcher Jean Baptiste Fourier and Swedish scientist Svante Arrhenius. Their identification of what came to be called the greenhouse effect (see box at right) applies to both natural and human-produced additions of CO2.

As measurements of atmospheric CO2 levels showed steady increases after World War II , Earth system scientists looked for a corresponding rise in global average temperatures, basing their studies on the physical laws governing the greenhouse effect. By the early 1980s, climate scientists were calling this atmospheric response global warming. Not every place on Earth was expected to warm at the same rate, and rising temperatures were not the only impacts anticipated. So some researchers began talking about global climate change to convey that the situation was more complex.

To some ears, "climate change" sounds less ominous than "global warming," so the question of how best to convey the seriousness of the problem continues. Suggestions include "global heating" and "global water crisis."

What is the average global temperature now?

Climatologists prefer to combine short-term weather records into long-term periods (typically 30 years) when they analyze climate, including global averages. Between 1961 and 1990, the annual average temperature for the globe was around 57.2°F (14.0°C), according to the World Meteorological Organization. According to estimates by the World Meteorological Organization, in 2008 that the global temperature was about 0.56°F (0.31°C) above that long-term average.

Why are global temperatures usually expressed this way—as a departure from normal, instead of a simple global temperature? One reason is that there are several different techniques for coming up with a global average, depending on how one accounts for temperatures above the data-sparse oceans and other poorly sampled regions.

Since there is no universally accepted definition for Earth’s average temperature, several different groups around the world use slightly different methods for tracking the global average over time. The important point is that the trends that emerge from year to year and decade to decade are remarkably similar—more so than the averages themselves. This is why global warming is usually described in terms of anomalies (variations above and below the average for a baseline set of years) rather than in absolute temperature. A Web site from NASA's Goddard Institute for Space Studies goes into more detail on the topic of The Elusive Absolute Surface Air Temperature.

How much has the global temperature risen in the last 100 years?

Averaged over all land and ocean surfaces, temperatures have warmed roughly 1.33°F (0.74ºC) over the last century, according to the Intergovernmental Panel on Climate Change (see page 2 of the Summary for Policymakers in the IPCC’s 2007 Synthesis Report). More than half of this warming—about 0.72°F (0.4°C)—has occurred since 1979. Because oceans tend to warm and cool more slowly than land areas, continents have warmed the most (about 1.26°F or 0.7ºC since 1979), especially over the Northern Hemisphere.

The year 1998 was the warmest on record for the contiguous United States, followed closely by 2006 and 1934, according to the National Climatic Data Center. In 2008, the U.S. saw its coolest year in more than a decade. It was the first time since 1997 that the nation has been close to its 100-year average temperature (though 2008 was still slightly above that norm). The United States was actually one of the least-warm spots on Earth in 2008 when compared to local averages. The globe as a whole had its coolest year since 2000, but the global average for 2008 was still warmer than any year from 1880 to 1996, according to NCDC.

There are slight differences in global records between groups at NCDC, NASA, and the University of East Anglia. Each group calculates global temperature year by year, using slightly different techniques. However, analyses from all three groups point to the decade between 1998 and 2008 as the hottest since 1850.

This graph from NOAA shows the annual trend in average global air temperature in degrees Celsius, through 2008. For each year, the range of uncertainty is indicated by the vertical bars. The blue line tracks the changes in the trend over time. Click here or on the image to enlarge. (Image courtesy NOAA's National Climatic Data Center.)

How much carbon dioxide (and other kinds of greenhouse gas) is already in the atmosphere?

One of the strongest pieces of evidence for human-induced climate change is the consistent rise in carbon dioxide (CO2) in modern times, as measured at the Mauna Loa Observatory in Hawaii, where CO2 has been observed since 1958. As of December 2008, the concentration of CO2 in Earth’s atmosphere was about 386 parts per million (ppm), with a steady recent growth rate of about 2 ppm per year.

Because CO2 stays in the air so long, it becomes very well mixed throughout the global atmosphere. This makes the Mauna Loa record an excellent indication of long-term trends.

Current atmospheric concentrations of CO2 are about 30% higher than they were about 150 years ago at the dawn of the industrial revolution. According to the Scripps Institution of Oceanography, ice core reconstructions going back over 400,000 years show concentrations of around 200 ppm during the ice ages and about 280 ppm during the warm interglacial periods. In other words, our current CO2 levels are higher than they've been in at least the last 400 millenia. See the Scripps Web site for a graphic illustrating this trend.

Almost a quarter of the carbon dioxide emitted by human activities is absorbed by land areas; another quarter is absorbed by the ocean. The remainder stays in the atmosphere for a century or longer.

Carbon dioxide accounts for more than half of the human-produced enhancement to Earth’s greenhouse effect. Among the other gases involved is methane, which has increased dramatically over the last century. Methane concentrations rose about 1% a year in the 1980s, but since about 2000 the concentration has leveled off, though a rise was observed in 2007. The reasons for this slow growth in recent years are not yet clear, although one possibility is a drop in the amount of methane leaked from natural gas pipelines and plants. Methane stays in the atmosphere for much less time than carbon dioxide (around a decade) and there is much less of it, but molecule for molecule, it is a far more powerful greenhouse gas. As of 2008, the concentration of methane in Earth’s atmosphere was about 1786 parts per billion.

Other important greenhouse gases include nitrous oxide and near-surface ozone. Water vapor is actually the most prevalent greenhouse gas, but human activity has not directly increased its concentration in the atmosphere, unlike the other chemicals above. However, as global temperatures increase, more water vapor is released by oceans and lakes, and this in turn helps to increase temperatures further. This is one of many feedback loops that help to reinforce and intensify climate change.

Hasn't the amount of carbon dioxide in the atmosphere decreased recently?

People don’t always produce more CO2 from one year to the next. When the global economy weakens, emissions from human activities can actually drop slightly for a year or two. Yet the accumulation of CO2 in the atmosphere continues to rise, as shown in the graph above. It’s a bit like a savings account: even if your contributions get smaller in a tight budget year, the total in your account still goes up.

Vegetation also makes a difference, because growing plants absorb CO2. Large-scale atmospheric patterns such as El Niño and La Niña bring varying amounts of flooding, drought, and fires to different regions at different times, which affects global plant growth. Thus, the amount of human-produced CO2 emissions absorbed by plants varies from as little as 30% to as much as 80% from year to year. Over the long term, just over half of the CO2 we add to the atmosphere remains there for as long as a century or more. About 25% is absorbed by oceans, and the rest by plants. This "balance sheet" is known as the global carbon budget.

It’s not yet clear which forests absorb the most CO2. Because the answer will influence global planning and diplomatic agreements on climate, scientists are working hard to measure how CO2 varies by latitude, altitude, and season. One such study is HIPPO, a field project led by NCAR and colleagues from 2009 to 2011 to take pole-to-pole measurements aboard an airborne laboratory, the NSF/NCAR Gulfstream V jet. Satellites such as Japan's GOSAT and others on the drawing board at NASA will help fill in more carbon-budget details.

What does the ozone hole have to do with climate change?

There are a few connections between the two, but they are largely separate issues.

First, it's important to know that ozone plays two different roles in the atmosphere. At ground level, "bad ozone" is a pollutant caused by human activities; it's a major component of health-damaging smog. The same chemical occurs naturally in the stratosphere, and this "good ozone" acts as a shield, filtering out most of the ultraviolet light from the Sun that could otherwise prove deadly to people, animals, and plants.

The ozone hole refers to the seasonal depletion of the ozone shield in the lower stratosphere above Antarctica. It occurs as sunlight returns each spring, triggering reactions that involve chlorofluorocarbons (CFCs) and related molecules produced by industrial processes. These reactions consume huge amounts of ozone over a few weeks' time. Later in the season, the ozone-depleted air mixes with surrounding air and the ozone layer over Antarctica recovers until the next spring. Other parts of the globe have experienced much smaller losses in stratospheric ozone.

Because of international agreements to limit CFCs and related emissions instituted with the Montreal Protocol, it's expected that the ozone hole will be slowly healing over the next few decades.

The ozone hole does not directly affect air temperatures in the troposphere, the layer of the atmosphere closest to the surface, although changes in circulation over Antarctica related to the ozone hole appear to be changing surface temperature patterns over that continent. Ozone is actually a greenhouse gas, and so are CFCs, meaning that their presence in the troposphere contributes slightly to the heightened greenhouse effect. The main greenhouse gas responsible for present-day and anticipated global warming, however, is carbon dioxide produced by burning of fossil fuels for electricity, heating, and transportation.

Higher up, the loss of stratospheric ozone has led to some cooling in that layer of the atmosphere. An even larger effect comes from carbon dioxide, which acts as a cooling agent in the stratosphere even though it warms the atmosphere closer to ground level. This paradox occurs because the atmosphere thins with height, changing the way carbon dioxide molecules absorb and release heat. Together, the increase in carbon dioxide and the loss of ozone have led to record-low temperatures recently in the stratosphere and still higher up in the thermosphere. Far from being a good thing, this cooling is another sign that increasing levels of carbon dioxide are changing our planet's climate.

Aren’t the computer models used to study climate really simplistic?

Global climate models—the software packages that simulate the past, present, and future of our atmosphere—have grown in complexity and quality over the last 10 to 20 years. Yet even the earliest models of the 1960s, which were quite crude by today’s standards, showed that a doubling of carbon dioxide in the atmosphere could increase global temperature by around 5°F (3°C). That projection remains close to the modern consensus, and temperatures over the last 30 years have risen at a rate consistent with this early estimate.

Far more information is available from today’s models, such as the NCAR-based Community Climate System Model, because they now include many more aspects of the Earth system, including ice sheets, vegetation, cloud areas, and soil moisture.

Research conducted for the 2007 IPCC Working Group 1 assessment compared the output from major models at research centers around the world. While these models are far from perfect, scientists are confident that they capture the key processes that drive climate. For example, models now replicate the ups and downs of 20th-century global temperature quite accurately.

As in other areas of science, rigorous testing and continual improvement are part and parcel of climate modeling. Researchers can test models against reality, identify and correct flaws, and compare their models with others.

Weren't scientists warning us about global cooling a few years ago?

After rising in the early 20th century, global surface temperatures cooled slightly from just after World War II (the mid-1940s) into the 1970s.

Scientists already knew that carbon dioxide was accumulating in the atmosphere and that it could lead to eventual global warming. In 1975, Wallace Broecker (Lamont-Doherty Earth Observatory) published the first major study with "global warming" in the title.

But a few researchers believed that pollution from burgeoning postwar industry was shielding sunlight and shading the planet, causing the observed cooldown. Some even theorized that a "snow blitz" could accelerate the cooling and bring on the next ice age. Their statements got major play in the media.

Starting in the 1970s, new clean-air laws began to reduce sulfates and other sunlight-blocking pollutants from U.S. and European sources, while greenhouse gases continued to accumulate unchecked. Global temperatures began to warm sharply in the 1980s and have continued rising since then.

Increasingly detailed models suggest that the more recent warmup can be attributed to greenhouse gases overpowering the effect of sunlight-shielding pollution. Computer simulations also suggest that today's atmosphere would be even warmer still, were it not for that air pollution.


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