Down to Earth

Global Climate Change Education

Weather is what's happening in the atmosphere on any given day, in a specific place. Local or regional weather forecasts include temperature, humidity, winds, cloudiness, and prospects for storms or other changes over the next few days. (Learn more about how NCAR studies weather.)

Climate is the average of these weather ingredients over many years. Some meteorologists like the saying that "climate is what you expect; weather is what you get," memorable words variously attributed to Mark Twain, Robert Heinlein, and others.

What's the difference between global change and climate change?
While global change and climate change are often used interchangeably, global change encompasses broader changes to all aspects of our world including areas such as the availability of water resources, sea-level rise, ocean acidification, and biodiversity.

Climate change is used to emphasize the specific changes most commonly associated with the atmosphere and the "average weather," including temperature, humidity, cloudiness, or precipitation changes.

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



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.)

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 IPCC's Climate Change 2007: Synthesis Report Summary for Policymakers (PDF)). 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.

If climate changes naturally over time, why isn't the current warming just another natural cycle?
The industrial activities that our modern civilization depends upon have raised atmospheric concentrations of carbon dioxide and methane to higher levels than at any point during the last 650,000 years. Scientists agree it is very likely that most of the global average warming since the mid-20th century is due to the human-induced increases in greenhouse gases, rather than to natural causes.

While natural variations have altered the climate significantly in the past, it is very unlikely that the changes in climate observed since the mid-20th century can be explained by natural processes alone.

What is the greenhouse effect?
This refers to the retention of the sun's warmth in Earth's lower atmosphere by greenhouse gases, which behave somewhat like the glass in a greenhouse.

These gases - primarily carbon dioxide, methane and nitrous oxide - act as a thermal blanket for the planet, warming the surface to a life-supporting average of 59 degrees Fahrenheit (15 degrees Celsius). The recently observed climate change is attributed to an accelerated greenhouse effect, caused by a boost in the levels of these gases in the atmosphere.

Are greenhouse gases increasing?
Human activity has been increasing the concentration of greenhouse gases in the atmosphere (mostly carbon dioxide from combustion of coal, oil, and gas; plus a few other trace gases). There is no scientific debate on this point. Pre-industrial levels of carbon dioxide (prior to the start of the Industrial Revolution) were about 280 parts per million by volume (ppmv), and current levels are greater than 380 ppmv and increasing at a rate of 1.9 ppm yr-1 since 2000. The global concentration of CO2 in our atmosphere today far exceeds the natural range over the last 650,000 years of 180 to 300 ppmv. According to the IPCC Special Report on Emission Scenarios (SRES), by the end of the 21st century, we could expect to see carbon dioxide concentrations of anywhere from 490 to 1260 ppm (75-350% above the pre-industrial concentration).

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 to the right. 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.

How much carbon dioxide 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 ofDecember 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.

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.

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.

Why does the temperature record shown on most records of climate change begin at 1880?
That happens to be when most of the land areas of the northern hemisphere started to report surface temperatures measured by thermometer to government weather authorities, who recorded and archived them. Scientists call this the “instrumental record” of climate. Prior to 1880, there are not enough of these instrumental records from enough of the world to produce estimates of global temperatures. Instead, temperatures from further back in the past are reconstructed from proxy records like tree rings, pollen counts, boreholes and ice cores. They’re less accurate, and because they’re fundamentally different kinds of data, scientists don’t generally put these proxy-based temperatures on the same chart as the instrument-derived temperatures.

Are El Niño and La Niña related to Global Warming?
El Niños are not caused by global warming. Clear evidence exists from a variety of sources (including archaeological studies) that El Niños have been present for thousands, and some indicators suggest maybe millions, of years. However, it has been hypothesized that warmer global sea surface temperatures can enhance the El Niño phenomenon, and it is also true that El Niños have been more frequent and intense in recent decades. Whether El Niño occurrence changes with climate change is a major research question.

Isn't the urban heat island effect (that elevates temperatures in large cities relative to rural areas) responsible for the measured increase in global temperature?
There is no urban heat island effect in the oceans, which cover nearly 3/4 of Earth's surface, yet sea surface and deep ocean temperatures are also increasing, consistent with what's occurring on land.

Is sea level rising?
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. Depending on which greenhouse gas increase scenario is used (high or low) projected sea-level rise is projected to be anywhere from 0.18 (low greenhouse gas increase) to 0.59 meters for the highest greenhouse gas increase scenario. However, this increase is due mainly to thermal expansion and contributions from melting alpine glaciers, and does not include any potential contributions from melting ice sheets in Greenland or Antarctica. Larger increases cannot be excluded but our current understanding of ice sheet dynamics renders uncertainties too large to be able to assess the likelihood of large-scale melting of these ice sheets.

Since it's snowing more on land-based ice sheets due to the warming, wouldn't this make sea level fall rather than rise?
It is indeed snowing more than it used to in the interiors of Antarctica and Greenland, because warmer air holds more moisture, but the mass gain in the interiors is more than offset by the mass loss around the margins due to glaciers sliding faster into the ocean in response to warming. So the net change is one of mass loss from the ice sheets, which is causing sea level to rise. This is responsible for about half of the observed 3 mm rise per year. The mass loss of Antarctica has been recently verified by the GRACE gravity satellite, which showed that gravity over Antarctica is weakening due to the loss of mass.

What does ozone layer depletion have to do with climate change?
Ozone depletion, which has produced "ozone holes" above Earth's poles, is caused by human-produced compounds that release chlorine and bromine gases in the stratosphere.

Ozone depletion does not contribute to global warming. Because ozone in the upper atmosphere absorbs heat radiating from below, stratospheric ozone depletion actually allows additional heat to escape into space. While this occurs worldwide, the depth of the springtime ozone hole over Antarctica results in Antarctica cooling while the rest of the Earth warms.

Greenhouse gases tend to cool the upper region of the atmosphere, where the ozone layer is located. Because the chemical reactions that cause ozone depletion happen more quickly in the presence of ice crystals, a colder upper atmosphere with more ice will probably cause the ozone hole to expand slightly.

What is the Intergovernmental Panel on Climate Change (IPCC)?
The IPCC is a scientific intergovernmental body set up by the World Meteorological Organization and by the United Nations Environment Program to provide the decision-makers and others with an objective source of information about climate change.

The IPCC brings together the world's top scientists in all relevant fields to provide reports based on scientific evidence and reflecting existing viewpoints within the scientific community. Click here to view the organization's most recent report series on global climate change.

How do we know what pre-industrial greenhouse gases and temperatures were?
Scientists have reconstructed past climate conditions through evidence preserved in tree rings, coral reefs and ice cores. For example, ice cores removed from 2 miles deep in the Antarctic contain atmospheric samples trapped in tiny air bubbles that date as far back as 650,000 years. These samples have allowed scientists to construct a historical record of greenhouse gas concentration and temperature stretching back hundreds of thousands of years.

If we stop all greenhouse gas emissions, will global climate change stop?
Industrial activity has already pumped billions of tons of greenhouse gases into the atmosphere, and we have yet to see the full effect of warming from those gases. A great deal of excess energy imbalance is stored in the ocean and will be released gradually over time, continuing to warm the planet.

In other words, some degree of climate change is irreversible. Scientists call this the "committed warming," and estimate that the Earth would continue to warm about 1 degree Fahrenheit (.6 degrees Celsius) even if greenhouse gas levels in the atmosphere stopped growing immediately. That is, if all human greenhouse gas emissions stopped tomorrow, the Earth would still warm for at least a half-century.

Weren't scientists warning us in the 1970s that an ice age was coming?
After rising in the early 20^th century, global surface temperatures cooled slightly from just after World War II (the mid-1940s) into the 1970s. These temperature drops were focused in the Northern Hemisphere.

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.

A few researchers believed that pollution from burgeoning postwar industry in North America and Eurasia was shielding sunlight and shading the planet, causing the observed cool down. 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. But the majority of scientists publishing in peer-reviewed journals were concerned that greenhouse gases would play a more dominant, warming role that would overtake the cooling of sulfate aerosol pollution in the coming decades. The state of climate science knowledge in the 1970s was summarized in a 2008 article on "The Myth of the 1970s Global Cooling Scientific Consensus" in the Bulletin of the American Meteorological Society (abstract | download PDF of full article).

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.

What is the role of the sun and solar cycles in climate change and global warming?
Since 1750, the average amount of energy coming from the sun either remained constant or increased slightly. Estimates of the amount of energy the sun has sent to Earth are based on sunspot records dating back more than two centuries, and other proxy indicators, such as the amount of carbon in tree rings.

More recently, satellite observation of solar activity from space suggest a slight increase in solar activity, but the change can't account for more than 10 percent of the warming trend seen during the past century.

What about all the news reports of errors and e-mail scandals in climate science?
A handful of errors in the 2007 IPCC Working Group II report were discovered in early 2010. They represent a tiny fraction of thousands of pages of findings in the complete report. To date, none of these errors has directly challenged the physical basis for detecting a human influence on Earth's climate.

The IPCC responded to these issues, as they report on their website. In March 2010, the United Nations and the IPCC asked the InterAcademy Council "to conduct an independent review of the IPCC’s processes and procedures to further strengthen the quality of the Panel’s reports on climate change." (IPCC, 2010 - PDF) The IAC is the umbrella organization for national academies of science around the world.

The IAC's report, released in August 2010, called the IPCC's process for producing assessments "successful overall," while recommending changes in management structure and procedures going forward, including greater transparency and communication.

Other concerns were raised in late 2009 when thousands of personal e-mail messages among several climate scientists were obtained without consent and posted on the Internet. UCAR and many other scientific organizations around the world responded to these concerns.

While the e-mail hacking incident raised a variety of issues, none of them directly challenged the major findings reported in climate change assessments, including the 2007 IPCC report.

What does NASA have to do with global climate change?
The agency's mission is to provide the scientific data needed to understand climate change and to evaluate the impact of efforts to control it.
NASA instruments, data, analysis and modeling contributed significantly the scientific reports on climate change issued by the United Nations Intergovernmental Panel on Climate Change (IPCC) - work that was awarded the Nobel Peace Prize in 2007.

Climate change and its effects are too uncertain. Why should we do anything?
Even the most conservative models show significant disruption. All decisions are made against a background of uncertainty. While it's true that scientists can't completely predict the future, that doesn't mean we should give up all attempts at planning according to our best knowledge. 

Scientists disagree. We don't know the science well enough yet, so why should we do anything?
Actually, there is strong scientific consensus on the reality of human-caused climate change. See the consensus/position statements of: - National Academy of Sciences - Intergovernmental Panel on Climate Change (IPCC) - American Geophysical Union (AGU) - American Meteorological Society (AMS) - American Association for the Advancement of Science (AAAS) Oreskes (Science, 2004) analyzed all abstracts in refereed scientific publications from 1993-2003 with the keywords "global climate change" (928 papers). None disagreed with the consensus position that human activities are causing the current warming.

Isn't there still a lot of debate among scientists?
The scientific method is built on debate among scientists, who test a question, or hypothesis, and then submit their results to the scrutiny of other experts in their field. That scrutiny, known as "peer review," includes examining the scientists' data, experiment and/or analysis methods, and findings.

The spirited debate around remaining uncertainties in climate science is a healthy indicator that the scientific method is alive and well. But the fundamental elements of climate change are not in dispute. To take just a few examples, we understand

Is there anything I can do to about global climate change?
A list of external resources with suggestions for individual action is provided in the Resources section of this website.

What about the future?
Due to the enormous complexity of the atmosphere, the most useful tools for gauging future changes are 'climate models'. These are computer-based mathematical models which simulate, in three dimensions, the climate's behavior, its components and their interactions. Climate models are constantly improving based on both our understanding and the increase in computer power, though by definition, a computer model is a simplification and simulation of reality, meaning that it is an approximation of the climate system. The first step in any modeled projection of climate change is to first simulate the present climate and compare it to observations. If the model is considered to do a good job at representing modern climate, then certain parameters can be changed, such as the concentration of greenhouse gases, which helps us understand how the climate would change in response. Projections of future climate change therefore depend on how well the computer climate model simulates the climate and on our understanding of how forcing functions will change in the future.

The IPCC Special Report on Emission Scenarios determines the range of future possible greenhouse gas concentrations (and other forcings) based on considerations such as population growth, economic growth, energy efficiency and a host of other factors. This leads a wide range of possible forcing scenarios, and consequently a wide range of possible future climates.

According to the range of possible forcing scenarios, and taking into account uncertainty in climate model performance, the IPCC projects a best estimate of global temperature increase of 1.8 - 4.0°C with a possible range of 1.1 - 6.4°C by 2100, depending on which emissions scenario is used. However, this global average will integrate widely varying regional responses, such as the likelihood that land areas will warm much faster than ocean temperatures, particularly those land areas in northern high latitudes (and mostly in the cold season). Additionally, it is very likely that heat waves and other hot extremes will increase.

Precipitation is also expected to increase over the 21^st century, particularly at northern mid-high latitudes, though the trends may be more variable in the tropics, with much of the increase coming in more frequent heavy rainfall events. However, over mid-continental areas summer-drying is expected due to increased evaporation with increased temperatures, resulting in an increased tendency for drought in those regions.
Snow extent and sea-ice are also projected to decrease further in the northern hemisphere, and glaciers and ice-caps are expected to continue to retreat.


Sources:
http://www2.ucar.edu/climate/faq
http://climate.nasa.gov/climatechangeFAQ/
http://www.ncdc.noaa.gov/oa/climate/globalwarming.html
http://www.ipcc.ch/pdf/assessment-report/ar4/wg1/ar4-wg1-faqs.pdf