Evidence for Climate Change

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In this lesson we'll examine some of the indicators that earth is warming and the evidence that suggests that these changes a result of human actions. We have two learning objectives. First, we'll discuss why many people think that recent climate changes are due to human activity; and, second, we'll review some natural indicators of a changing climate. But before we can begin, we have to explain the term that is used a lot during discussions to climate change, We need to introduce you to a temperature anomaly. A temperature anomaly represents the difference between the measured temperature and a long-term average value. Climate researchers calculate long-term averages from data sets spanning thirty years or more. For example this graph shows average annual global temperatures for each year since 1880. Rather than plot the actual temperature, the data are compared to the long-term average calculated for the last century.

The zero line represents the calculated average temperature, blue and red bars show the difference above or below average for each year. The red bars indicate positive temperature anomalies; the blue bars are negative anomalies. Just on the basis that this data, you can see that it looks like temperatures have been getting warmer in recent years but you wouldn't be the only one to think that. If we were to analyze thousands of scientific papers that discuss the relationship between climate change and human activity, we would find that 97 percent of them make the case that humans are causing changes that result in the warming of Earth. In contrast, if you were to pose a similar question to the general public, you would find just 41 percent of people agreeing that climate change was happening and that it is caused by human actions. A lot of other folks either don't know or disagree.

So, first let's look at some of the evidence that has convinced scientists that we humans have caused global warming and then let's examine some changes to the earth system that support the interpretation of a warming Earth. If we compare temperature anomalies over the last 1000 years we can see that recent temperatures have not been matched in the last millennium. The rate at which temperature changed and the length of the warming interval in the 20th century are greater than at any other time in the recent past. The reason for this temperature spike is related to a dramatic increase in the concentration of greenhouse gases, like carbon dioxide, in Earth's atmosphere. For much of the last million years carbon dioxide concentrations were below 300 parts per million.

But these values have risen dramatically as human population increased. Since the industrial revolution the concentration of co2 in the atmosphere has increased by 40 percent. The science linking increasing concentrations of greenhouse gases to higher temperatures is well established among atmospheric chemists. Consequently, it should be no surprise that since 1850, the globally averaged surface temperature rose by 0.85 degrees Celsius or 1.5 degrees Fahrenheit or that the 10 hottest years on record have all occurred since 1998. One reason why people find it difficult to make up their minds about global warming is that their views on climate trends may be influenced by short term weather phenomena; but we need to understand that these types of seasonal or annual fluctuations are due to natural variability within Earth's climate system.

We must remember too, that just because the global average temperature is increasing; some parts of the world will experience more or less warming than average. These spatial variations in temperature are represented on this map which shows which part the world were hotter or cooler than average in 2013. While most the world was warmer than average as indicated by the amount of red and pink on the map, temperatures across much of North America were actually below average during 2013. If global temperatures really are warming for good, then these changes should affect many elements of the Earth system. Let's concentrate on two topics for which we have a lot of evidence; the distribution of snow and ice, and the characteristics of the world's oceans. As you might expect, with increasing temperatures we see a decline in the volume and areal coverage of sea ice. The Northwest Passage, a cheaper faster shipping route from the Atlantic to the Pacific Oceans, may soon become passible during Arctic summer for the first time in human history. About one-third of Earth's land surface, almost all of it in the Northern hemisphere, is covered by snow for some part of each year. Across the Northern hemisphere the total area covered by snow during March and April is less than in the past decades and it is melting earlier than it did previously.

Since the late '80's there have been twice as many years when snow cover was below average as when it was above average. The total volume of glacial ice on Earth has declined steadily for several decades. The 1980 photograph on the top left shows the end of the nearly 50 meter high Muir glacier in Alaska. The lower image taken 23 years later documents that the glacier has retreated more than two kilometers and it has thinned by several hundred meters. The graph in the lower right shows the cumulative mass balance of three US Geological Survey benchmark glaciers from Alaska and Washington. Negative values indicate a net loss of ice and snow compared with their condition in 1965. Some of this melting of glacial ice is ending up in the oceans and contributing to rising sea levels. But the main cause of the rising oceans is a thermal expansion of warmer ocean waters.

The graph shows the global sea-level anomaly as measured by tidal gauges and satellites. During the last century the average sea level has risen by 1.7 millimeters per year or about an inch every fifteen years. The temperatures of the waters in the shallow ocean are increasing, just like temperatures on land. Changes in sea surface temperature vary regionally. Much of the world's oceans have seen surface temperatures increasing by 1-2 degrees Fahrenheit since 1901. Some areas have increased by more while other areas have actually cooled. August 2014 was the world's hottest August since record-keeping began in 1880. Data suggest that this pattern will be repeated in future years. While there may be cooler years or region's, the general trends are for higher temperatures and sea levels and for less snow and ice. We had two learning objectives, how confident are you that you can successfully complete these tasks..

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