|MadSci Network: Earth Sciences|
Thank you for writing to the MadSci Network. You have asked many individual questions - I will try to answer them all.
Global warming is defined as an increase in the average temperature of the Earth’s atmosphere and oceans. There has been a measured increase of about 1° C in the global average temperature since the late 1900s. This is not seriously debated by climate scientists. While there are many factors (i.e solar, volcanic, Earth’s orbit, water vapor, carbon dioxide, methane) that can influence temperature on both local and global scales, only very few natural factors can possibly explain this significant increase. A very large majority of scientists think that the only way to explain it is because human activities have dramatically increased the amounts of greenhouse gases such as carbon dioxide in the atmosphere over the past 100 years. This conclusion has been based on comparisons between measurement records of temperature (which in the case of ice core data go back 800,000 years!) and detailed computer models of climate processes. These models are often called global climate models, or GCMs.
Please note that carbon dioxide is emitted by natural processes as well as
human ones, but there is clear evidence to prove that the large increases
observed in the past 100 years come specifically from humans. (Please see
the following link for more info:
Climate has varied much over Earth’s history, even prior to human industrialization. Glacial (relatively cold) and interglacial (relatively warm) periods are one type of climate variation you may have heard of. We are currently in an interglacial period. These events have occurred on time scales of tens of thousands of years, and were primarily caused by one of the things I think you were referring to in your mail - changes in the Earth’s orbit around the sun. The orbital changes are called Milankovich cycles. Other variations can occur on larger or smaller time scales. For example, there is an 11 year sunspot cycle that affects the amount of energy the sun releases, and therefore affects the amount of energy entering Earth’s atmosphere.
These and many other natural variations are included in the detailed GCM calculations, but on their own can not account for the large amount of warming that has occurred over the last 100 years. However, the known increases in emissions of carbon dioxide and other greenhouse gases can explain the amount of observed warming. In addition, the spatial, altitudinal, and time patterns of temperature changes are more similar to those expected to result from increased carbon dioxide emissions than for any other potential influencing factor. So to say it more simply, the only way that the computer models correctly simulate the measured temperature trends of the last 100 years is if we include the known (measured) emissions of carbon dioxide and other greenhouse gases released into the atmosphere from human activity.
It’s important to mention that the factors you mentioned do dramatically influence temperatures on a smaller, and more familiar, scale. Earth’s daily rotation causes day and night with typical temperature changes of 10-20 degrees or more, depending on the location. The tilt of the earth (meaning that the North Pole is sometimes tilted towards the sun, and sometimes tilted away from it) defines the seasons, which in turn creates even larger temperature changes at a given location. It can be easy to confuse these types of temperature changes with the idea of global warming. Remember that global warming is meant as the Earth’s average temperature, which means that the very hot and very cold regions (and their daily and seasonal changes) tend to balance each other out. While a 1° C global average temperature change may not seem very large, the last Ice Age (when the Northeast US was covered with 3000 ft of ice ~10,000 years ago) was only 5° C cooler than today!
You also refer to the large amount of recent media attention that has been focused on links between global warming and hurricanes. Hurricanes are caused when the ocean surface is warm enough to have large amounts water evaporates from it. Once in the atmosphere the water eventually condenses to form clouds and rain, and warms the surrounding air as a result. The hot temperatures create a region of low pressure, which causes winds to spiral and form a hurricane.
Many atmospheric scientists and meteorologists have spent much time looking at past records to see if there are statistical links between the measured temperature increases and hurricane frequency and intensity. Evidence is accumulating that global ocean warming is causing the hurricanes that do form to have a stronger intensity than they would have had if the ocean temperatures were cooler. However, the evidence does not show that the number of hurricanes has been affected by global warming.
I hope this helps!
Please also refer to the following websites for more information: http://www.ngdc.noaa.gov/paleo/ctl/index.html
MadSci Answer: 850148721.En
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