Topics : Climate Change

Earth's Past | Present Greenhouse Warming | Earth's Future | El Niño | Drought & Wildfire

Climate of the Future

flooded streets in Florida

Coastal population centers at low elevations like New Orleans, Key West, and other spots around the world, are vulnerable to sea level rise. Florida's Key West floods routinely in springtime when tides pulled higher by the new or full moon reach 1.7 feet above mean sea level. (Photo courtesy National Weather Service Forecast Office, Key West; NOAA Photo Library.)

Why should Earth be warming? The amount of energy emitted by the Sun has risen a fraction of a percent since 1900. However, climate simulations at NCAR have shown that solar changes explain less than a third of the warm-up during the last century. The most straightforward explanation for a warming Earth is the greenhouse gases emitted when fossil fuels are burned in homes, gas and coal-fired power plants, vehicles, and factories.

Water vapor, carbon dioxide (CO₂), and other greenhouse gases act to warm Earth’s atmosphere by absorbing, and trapping, some of the outgoing radiation from the Earth and reradiating some of this energy back to the surface. The amount, or concentration, of CO₂ gas in the atmosphere has risen more than 30% since widespread fossil-fuel use began with the start of the industrial revolution in the late 1700s. CO₂concentration in the atmosphere is now at its highest point in more than 600,000 years.

Each year, the CO₂ concentration increases by about 0.5%. Because CO₂ has a lifetime in the atmosphere of over 100 years, its atmospheric concentration will continue to increase as long as emissions from human activities continue.

Many NCAR scientists are part of a global team studying this problem and its meaning for our planet's future. The Intergovernmental Panel on Climate Change (IPCC) includes more than 1,000 experts from a variety of climate specialties. The next IPCC report will be issued in early 2007.

In their previous report (2001), the IPCC predicted that increasing levels of greenhouse gases will warm the globe by a significant amount. The most probable range, according to the IPCC, is between 2.5 and 10.5°F (1.4–5.8°C) over 1990 levels by the year 2100. Also in 2001, an NCAR scientist and his colleague estimated a 90% likelihood that the range will fall between 3 and 9°F (1.7–4.9°C).

world map of change in frost days

The average number of frost-free days per year is projected to rise across much of the globe by the 2080s, with the largest increases (red and orange) across the western fringes of North America and Europe. (Illustration courtesy Gerald Meehl and Claudia Tebaldi, NCAR.)

What does this mean for society? A vast majority of climate scientists agree with the IPCC consensus that Earth will warm along with increasing greenhouse gases. However, the effects will be far more varied than a simple and uniform warming over the entire planet, because heating also alters the water cycle, among other changes. As a result, some regions will become considerably hotter or cooler, or wetter or drier, than others.

Several national studies have addressed these regional consequences, including those for the United States, Canada, and the United Kingdom. Some aspects of regional climate change are already well established. For instance, high-latitude areas such as Canada, Russia, and the Arctic are warming more rapidly than the tropics, as predicted by computer models. This trend is expected to continue. In many nations, rainfall and snowfall are becoming more concentrated in heavier bursts, and regions poleward of latitude 40 degrees north are expected to see more days with heavy precipitation. NCAR scientists and colleagues are working to improve understanding of other potential regional changes in climate, such as where U.S. rainfall and snowfall patterns might shift.

Researchers are also working to improve techniques for assessing the impacts of a changing climate on environment and society. One method is to translate temperature changes from a model into trends that affect people's everyday lives. A 2004 NCAR study found that, by the period 2080-99, American and European heat waves will be more severe, frequent, and long-lasting.

A related study found that frost days (those in which temperatures dip to 0°C or 32°F) will decline in many parts of the globe by 2080-99. The largest decreases are projected across the northwest parts of Europe and North America, as mild marine air becomes more prevalent in winter. Such a change would affect agriculture and tourism as well as natural ecosystems.

New research in 2006 by NCAR scientists and colleagues looked more specifically at the potential for an increase in weather extremes in a warmed climate. The researchers used simulations from nine different climate models to demonstrate the risk of dangerous heat waves, intense rains, and other kinds of extreme weather in the next century.

This map portrays a statistical analysis from about 20 general circulation models that ran experiments on future climate projections. For 23 regions of the globe, each bar's color range corresponds to temperature in degrees Celsius (see legend at bottom left). Each horizontal segment corresponds to a probability, with the far right segment on each bar equalling the amount of temperature change that the corresponding region will experience with 95% probability during winters at the end of the 21st century.

For each region two bars are shown, corresponding to two different scenarios regarding the emissions of greenhouse gases. The top bar represents a pessimistic scenario that assumes high greenhouse gas emissions in the future (IPCC Scenario A2), while the bottom bar represents a more optimistic scenario of lower emissions (Scenario B1). Compared to the top bars, the value ranges of the bottom bars are lower, representing less warming. Still, even under the optimistic scenario, there is a 95% probability that all the regions of Earth will experience a temperature increase. The warmest colors on the map are located in the high-latitude regions of the northern hemisphere, where scientists expect the greatest warming to occur.

The ranges of uncertainty illustrated by the bars should be regarded as fairly conservative. The particular models used in the study have a relatively constrained view of climate's sensitivity to the effects of future greenhouse gas emissions. The study also assumes that climate's natural variability will remain the same as it is now, whereas other studies have argued that natural variability may be greater in the future. (Analysis and illustration by Claudia Tebaldi et al., NCAR.) Click here or on the image to enlarge the map.

These three studies, along with many others, have been used in preparing the 2007 IPCC assessment. One theme for the new assessment will be the extent to which our planet is committed to some amount of human-induced climate change, regardless of what actions we take in the future.

Forecasting regional climate poses a challenge similar to that faced by weather forecasters every day. Forecasters may call for a 70% chance of rain with confidence, even if they cannot say where each thunderstorm will form. In much the same way as that forecast for a chance of rain, scientists are finding probabilities a useful way to describe how regional climate will change in a warming world. Long-range climate outlooks may give the chance of a week of 100-degree weather, for example, and how that chance would increase as the decades unfold. NCAR scientists are developing methods to determine the probabilities of future changes to the climate on regional scales.

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