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How serious is global warming of a few degrees? Couldn't warming be beneficial?A warming of 1.1°F over the past century and a further 2.5-10.4°F over the 21st century, as projected by IPCC, may appear minor compared to short-term weather changes from night to day and winter to summer. But in global climate terms, a warming at this rate would be much larger and faster than any of the climatic changes over the past 10,000 years. Global temperatures during the last ice age (about 20,000 years ago) were "only" 9°F cooler than today, but that was enough to allow massive ice sheets to reach as far south as the Great Lakes and New York City.
The warming that humans are causing will change Earth's climate in the opposite direction, but tens or possibly a hundred times faster than natural rates of climate change. Warming of a few degrees would lead to more frequent droughts and heat waves, cause greater rainfall, and possibly change the strength of storms.
It is possible that some areas would benefit from global warming even as other areas were harmed. Certain farming areas, for example, could enjoy a longer growing season, while others suffer from more frequent droughts. Local impacts, however, are the most difficult to predict, making it difficult to know who will benefit and who will not, and for how long these conditions will persist, as the warming continues and the climate keeps changing.
How can we talk about climate change over the next 100 years when we can't be sure of tomorrow's weather?Weather and climate are different. The methods used to forecast changes in weather and climate differ as well. Because the weather changes day to day, current weather forecasts are reliable for roughly ten days. Climate, on the other hand, can be thought of as average weather, including weather's variability over much longer time horizons (e.g., from year to year).
Natural changes in our planet's climate happen over the course of years, centuries and many millennia. Long-term climate forecasts are possible because scientists understand many of the factors that influence climate over such long periods, such as changes in the sun's energy and the level of greenhouse gases in the atmosphere.
Climate scientists do not claim to know how to predict day-to-day fluctuations (weather) over the 21st century. Rather, they are predicting how they think average temperature and precipitation (climate) will change due to human activities.
How do we know CO2 in the atmosphere is increasing? And that humans are responsible?Careful measurements have confirmed that CO2 is increasing in the atmosphere and that human activities are the primary cause. CO2 measurements have been taken directly from the atmosphere over the past few decades. CO2 trends for earlier times have been derived from measurements of CO2 trapped in air bubbles in glacial or polar ice.
The 31% increase in atmospheric CO2 observed since pre-industrial times cannot be explained by natural causes. CO2 concentrations have varied naturally throughout Earth's history. However, CO2 concentrations are now likely higher than any seen in at least 420,000 years.
Aren't other factors responsible for global warming?Natural and human factors affect the average temperature of our planet. Natural variability in the Earth's climate system can cause changes over decades to centuries. Gradual changes in Earth's orbit around the sun (which in turn change how sunlight hits our planet) are thought to be the key pacemaker for the comings and goings of past ice ages over many millennia.
The sun's energy can also vary slightly over time. Large volcanic eruptions can cool the planet for a few years by spewing out particles that block out some sunlight. Even some of our own pollutants, like the sulfur dioxide released from power plants and heavy industry that contributes to acid rain, have a similar cooling effect.
Depletion of the ozone layer – caused by our release of chlorofluorocarbons – has led to cooling of the upper atmosphere. Scientists think these temporary cooling effects have been masking some of the long-term warming being caused by human emissions of greenhouse gases. Over the 21st century, the ongoing buildup of greenhouse gases in the atmosphere is likely to be the most dominant influence on our planet's climate.
How will global warming affect the polar ice caps?Polar ice caps are some of the largest surface features on our planet and any changes to them, however small, could have far-reaching effects. Melting due to global warming is expected to reduce the size and extent of the polar ice caps, even after taking into account the potential for more snow and ice accumulation atop the ice sheets due to increased precipitation.
Melting of polar ice and land-based glaciers is expected to contribute to the 3.5 inch to three feet of sea level rise projected by the IPCC for the 21st century. Shrinking ice caps may also cause changes in ocean circulation and even storm tracks. To be sure, not all of the melting currently occurring is due to global warming, and the melting of floating sea ice does not affect sea level.
Further warming will likely accelerate the shrinkage of ice caps and glaciers, however. Of particular concern is the stability of the West Antarctic Ice Sheet. A sudden collapse would raise sea levels 16-20 feet but the IPCC considers the likelihood of such a collapse before the year 2100 low.
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