Impacts of climate change on the natural world In the previous - TopicsExpress



          

Impacts of climate change on the natural world In the previous article we looked at the instrumental temperature record of global mean climate over the past 150 years. We saw that, over that period, the global mean near-surface temperature has increased by 0.9˚C. When stated as a number like this it may not sound like very much and so in this article we’ll look at what impacts this change has had on just a few elements of the natural world. By looking at the impacts of climate change rather than just considering a single value of temperature change, we can see more easily how climate change is already affecting our planet. The first and absolutely crucial thing to realise about the observed warming is that it has not been uniform across the globe. Regional analysis of the temperature record shows that the warming has been greatest at high latitudes, especially in the Northern Hemisphere, and over land rather than over the oceans. We would expect the impacts to be felt most strongly in regions where the warming has been largest. source: imgur Figure 1. A graph of Northern Hemisphere sea ice extent during September, from 1979-2014. Data from the US National Snow and Ice Data Center. The Northern Hemisphere high latitudes are dominated, at least in winter, by extensive sea ice coverage, with the Arctic Ocean being entirely covered by ice in winter, with ice also extending into the northern-most parts of the Atlantic and Pacific Oceans. This sea-ice coverage reaches a minimum extent in September before re-establishing through the Autumn and Winter. We have reliable satellite estimates of the extent of this ice coverage going back to 1979 and figure 1 shows the trend in the sea ice extent in the Northern Hemisphere. Since 1979 we have seen a reduction in sea ice extent during September of 13.7% per decade. Whilst there are very large year-to-year fluctuations in the sea ice extent, this downward trend is very robust and equates to a loss of well over one million square kilometres of ice. This sea ice plays a small but important role in the Earth’s radiation balance. Ice is white and so acts to reflect a large portion of incoming solar radiation that falls upon it back out to space. Replacing this highly reflective ice with dark sea-water in the Summer months means that some of the incoming energy from the Sun that used to be reflected to space is now absorbed into the oceans and retained as heat within the Earth system. In the southern hemisphere the story is quite different and Antarctic sea ice extent has actually been increasing over the same period. Ice sheets on land (known as glaciers) are also being affected by increases in global mean temperature. Since 1945, scientists have been monitoring the extent of an increasing number of major glaciers around the world. A set of about 40 ‘reference’ glaciers are used as indicators of the global trend, and this set indicates that, on average, the equivalent about 25 metres of thickness has been lost from these glaciers since 1945. This has been happening at different rates in different glaciers and whilst many are receding rapidly, a few are actually growing. Another effect of global warming on the natural world that is already becoming apparent is sea level rise. Naively one might expect this rise to be due to water from melting ice ending up in the sea, but actually the bulk of the rise in sea level associated with climate change comes through expansion of sea water as the oceans warm up (warmer water is less dense than cold water). Current estimates suggest that global mean sea level has risen by about 20cm since the 1850’s. Whilst this may not sound like very much, it is a global and annual average value, and when added to the heights of the highest spring tides or storm surges associated with strong winds, it means that coastal defences designed to protect low lying areas from coastal flooding may no longer be adequate. Whilst changes to sea and land ice and sea level may be quite big and dramatic there are many smaller, subtle shifts taking place in the natural world in response to changes in climate. The flowering dates of a wide range of plant species are becoming earlier as the temperature in Spring increases. A study using a combination of diaries of natural events kept since the 18th century has shown that flowering dates of a range of UK species averaged over the past 25 years have been between 2 and 13 days earlier than any other consecutive 25 year period since 1760. At high latitudes in the Northern Hemisphere boreal forests are gradually encroaching into the tundra and in the oceans, the distribution of fish stocks is changing in response to changes in ocean temperature.
Posted on: Sun, 23 Nov 2014 22:31:00 +0000

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