For example, to obtain temperature records from tree rings, scientists drill cores into several trees that are growing in a region. They identify site-specific factors that influence tree growth such as temperature, precipitation, altitude, and tree age, and then compare these factors against the width or density of the tree rings over the lifetime of the tree. Once researchers are confident about how local tree growth correlates to air temperature, they then seek out older trees in the region that are preserved perhaps in a local swamp or lake.
By matching rings from trees that partially overlap life spans, scientists can construct a continuous climate record over thousands of years. Ice cores have bands of light and dark areas with traces of various substances, which can be analyzed as to composition and age, yielding important information about the environmental conditions throughout time. Antarctic ice core records vividly illustrate that atmospheric carbon dioxide CO2 levels today are higher than levels recorded over the past , years Figure 4.
Atmospheric CO2 levels have risen 30 percent in the last years, with half of that rise occurring only in the last three decades. It is a well-established scientific fact that CO2 and other gases emitted from industrial and agricultural sources traps heat in the atmosphere, so it is no surprise that we are now witnessing a dramatic increase in temperature.
Is theory on Earth's climate in the last 15 million years wrong?
Much as the Air Force builds computer programs to simulate flight under different conditions, climate scientists build computer programs to help simulate global climate. These computer programs, called General Circulation Models GCMs , use various assumptions about physical, chemical, and biological processes that occur within Earth's atmosphere and oceans and on its land surfaces.
To ensure accuracy, each model is checked to see if it generates results that correctly reproduce the past and current climate. Once accuracy is established, the computer program can then be used to explore the likely future climate if, for example, we double the atmospheric concentration of carbon dioxide. Over the past million years, shifts in orbital cycles influenced the amount of sunlight falling on the northern hemisphere and played a major role in the waxing and waning of ice ages.
Whenever atmospheric carbon dioxide increased naturally either as a direct cause or as an amplifier, see this study , global surface temperature increased further. This held true even when local temperatures in the northern hemisphere and southern hemisphere might be out of phase due to factors including ocean circulation. Past climate evidence informs just how powerful carbon dioxide is. All else being equal, increases in atmospheric carbon dioxide lead to warmer global average surface temperatures.
That is a proven relationship known since at least and confirmed throughout the years. This is also true of the current warming trend--an increase in carbon dioxide is leading to an increase in temperatures. It was the first comprehensive study combining data from many different archives of temperature including tree rings, ice cores, and coral reefs. It demonstrated that Northern Hemisphere temperatures rose sharply during the late 20th century, in marked contrast to the relatively small temperature fluctuations during the previous six centuries. The graph got its name because its shape resembles a hockey stick, with the blade end representing the sharp temperature rise over recent years.
As is typical of the scientific process, independent teams of researchers have worked to reproduce the results of the "hockey stick" by using their own approaches and even by using slightly different data.
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Despite small differences, they still yield the same essential conclusion: the past to year period was likely the warmest of the past millennium. The hockey stick graph is a high-profile example among literally thousands of pieces of evidence that have contributed to the present scientific consensus on the human influence on global warming. We are observing real changes due to higher temperatures. Here are some examples from the AR Glaciers are melting.
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Sea levels and ocean heat content are rising Figure 7. Patterns of rainfall and snow , droughts and storms , and lake and sea ice are changing. Plant and animal distributions and interactions are shifting and changing , with potential severe effects on crop pests and food supplies. Compared with natural factors that influence climate including solar variation and volcanic eruptions , human activities — primarily burning fossil fuels and deforestation — have been a major contributor to climate change over the last 50 years.
Burning coal, oil and gas and destroying forests overloads the atmosphere with excess carbon dioxide, adding to heat-trapping gases that already are present in the atmosphere. Combined, these gases act like a blanket covering the earth. The human contribution to this effect is unmistakable : the part of the atmosphere where excess carbon dioxide accumulates has expanded and warmed dramatically in recent years precisely during the period when emissions from human activity have increased. But when it comes to climate the process is not quite the same.
While global temperatures have been going up overall, the rate of warming varies. As a result, some years are warmer than others.
That is because there are factors, like air and ocean circulation patterns, that affect both the rate and the intensity of the global warming. Imagine that you add a few external factors to that imaginary warming kettle a fan or air conditioning vent blowing directly at the stove; adding more water hot or cold to the kettle; a bucket of ice cubes sitting by its side; etc. Nevertheless, both years were still among the warmest years on record and would have been even hotter without this short-term cooling effect.
And needless to say, winters continue to be colder than summers, even though some are more or less cold. Oscillations between cooler and warmer periods are therefore expected, and do not change the long-term trend of warming shown by the data.
Focusing on the relatively short time periods where the rate of warming is slower in order to claim that global warming is not happening is a misleading way to use statistics, yet one that has been the focus of attention. If one starts to measure a warming trend from a much warmer year, the rate of warming is bound to be different from the long term trend, simply due to the starting point.
Climate is a good indicator of what to expect in general, such as cold days in February in New England. To start, build a stone mine and a woodcutting center by clicking the images in the bottom left corner, and then clicking an open space touching the ground. Then build houses to grow your population, and farms to make more food. Keep an eye on your resources and try to keep your colony fed and happy.
Play the classic game of strategy.
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You can challenge the computer, a friend, or join a match against another online player. Ever played the classic game Snake? Slither around and eat the apples to grow your snake as long as possible. But don't hit the wall, or eat your own tail! Activate the power bounce! Swing your way through each challenging course without hitting anything.
How many tries will it take you? You've got a limited number of slices to cut the wood into the correct number of pieces. Be precise when you slice! Hop in your helicopter and join this strategic multiplayer game. Specialize your copter, build a base, and conquer the map! Hey you, stop!
And you, drive!