Between the years 1600 and 2007, Earth's population grew to 6.07 billion people and fossil fuel use became widespread, increasing atmospheric carbon dioxide (CO2) from an estimated 274 parts per million to 385.
CO2 traps heat in the atmosphere, contributing to global warming, because it absorbs radiation from Earth's surface that would otherwise escape into space.
Earth's average temperature has risen about 1.8°F over the past 100 years and it will rise much more as long as CO2 emissions continue to increase at current rates.
Even if emissions were to stabilize today, temperature would continue to rise for several decades due to the delayed response of the climate system.
The years 1998, 2005, and 2007 are the warmest ever recorded according to historical weather data.
Over the past 50 years, the top 2,300 feet of the ocean has warmed about 0.18ºF. The ocean's response to climate change has been slower than the atmosphere. Therefore, because the ocean stores most of Earth's heat, ocean temperature and sea level will continue to rise for several decades due to past greenhouse gas emissions, even if emissions are curbed today.
Climate change means more extreme weather. Over the last 60 years, heavy rain and snow storms have become 24% more frequent in the U.S. Warmer ocean waters also may make strong hurricanes even more powerful.
The ocean is acidifying. About 30% of the CO2 released by human activities over the past 200 years has already been absorbed by the ocean. Ocean acidification is expected to make it harder for corals, plankton, and other shell-forming organisms to grow shells.
Deforestation contributes to global warming. Trees benefit the environment by absorbing CO2, but they return it to the atmosphere upon decaying or burning. Nearly 40% of Earth's land surface has been converted for human use; as a result, large quantities of CO2 have been released into the atmosphere-at the same time, fewer trees are left to absorb it.
Changing Climate = Changing Ice Changing Land
Today, ice sheets are melting much faster than they were only a couple of decades ago. Greenland, for example, is losing about 50 cubic miles of ice per year. If the Greenland ice sheet was to melt completely, for instance, global sea level would increase by 23 feet because the transfer of ice from land into the ocean makes sea level rise. Although sea level rise of this size may take centuries or longer to complete, hundreds of millions of people would eventually feel the impact.
Ice shelves are massive platforms of ice floating on the ocean, especially around Antarctica. Scientists have learned that floating ice shelves act as dams to glaciers, which are actually flowing rivers of ice. After Antarctica's Larsen B ice shelf broke up in 2002, glaciers behind the shelf began flowing into the sea much more quickly.
Sea level has risen about 7 inches over the last 100 years, mostly due to the expansion of water as it warms. Every foot of sea-level rise translates to 100 feet of shoreline loss on the Eastern U.S. coast. Predictions vary, but future sea-level rise could range from 7 inches to more than 40 inches by 2100.
The primary effects of sea-level rise are increased flooding during storm surges and coastal erosion and submergence. Some of the world's largest, most densely populated cities are located in these regions; indeed, 634 million people live within 33 vertical feet of sea level.
Of all the energy reaching Earth from the Sun, about two-thirds is absorbed by land, ocean, and atmosphere, and one-third is reflected back into space. Most of the absorbing takes place in the tropics, where dark, plant-covered land and dark ocean predominate. Much of the reflecting happens at the poles, with their bright coverings of snow and ice.
The last few decades have seen larger areas of the world enduring longer—and drier—droughts. One reason is that warmer soils lose their moisture faster, intensifying drought conditions. Also, warming tends to cause more intense but less frequent rainstorms and changes local weather patterns.
Many regions around the globe have seen a sharp increase in the number of wildfires over the past several decades because of climate change. In the American West, spring-summer temperatures averaged 1.6ºF higher between 1987 and 2003 than in the previous 15 years. That same span saw four times as many wildfires, many at higher elevations.
