Evidence of Global Warming During the Last Hundred Years

15.2.2.1 Climate Changes

Periods of earth warming and cooling occur in cycles. This is well understood, as is the fact that small-scale cycles last 20,000 years. Earth's climate was in a cool period from AD 1350 to about AD 1860, dubbed the "Little Ice Age" (Fig. 15.4). The decline in global temperature was only 0.5°C on average, but its effects were more pronounced at higher latitudes, dramatically characterized by drier climate, harsh winters, shorter growing seasons, and crop failures. Today, the global temperature has warmed to the levels of the former "Medieval Warm Period," which existed approximately AD 1000-1350 (Fig. 15.4). Over the last 400,000 years, the Earth's climate has been unstable, with significant temperature changes, ranging from a warm climate to an ice age as rapidly as within a few decades. These rapid changes suggest that the climate may be sensitive to internal or external climate forcing and feedbacks. As can be seen from the dashed-line curve in Fig. 15.5, temperatures have been less variable during the last 100,000 years. Based on the incomplete evidence available, it is unlikely that global mean temperatures have varied by more than 1°C in a single century during this period. The information presented in this graph indicates a strong correlation between CO2 content in the atmosphere and temperature. Here is a possible scenario: anthropogenic emissions of greenhouse gases (GHGs) could bring the climate to a state where it reverts to

Fig. 15.4 Variations in surface air temperature during the past 1,000 years (from the IPCC third report 2001). The recent "Little Ice Age," from about 1,350 to 1,860, shows modest global cooling (0.5°C average) but heavy effects at higher latitudes

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Fig. 15.5 Temperature and CO2 levels in the atmosphere over the past 400,000 years. The geological records clearly show a correlation between CO2 (from air bubbles from the Antarctic ice core) and temperature trough glacial/interglacial cycles

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Fig. 15.5 Temperature and CO2 levels in the atmosphere over the past 400,000 years. The geological records clearly show a correlation between CO2 (from air bubbles from the Antarctic ice core) and temperature trough glacial/interglacial cycles the highly unstable climate of the pre-Ice Age period. Rather than a linear evolution, the climate follows a nonlinear path with sudden and dramatic effects when GHG levels reach an as yet unknown trigger point.

15.2.2.2 Ocean Warming: Effects on Coastal Environments

Over the last 100 years, the global sea level has risen by about 10 to 25 cm. Sea level change is difficult to measure, and relative sea level changes have been calculated mainly from tide-gauge data. In the conventional tide-gauge system, the sea level is measured relative to a land-based tide-gauge benchmark. The major problem is that the land experiences vertical movements (e.g., from isostatic effects, tectonic movements and sedimentation), and these movements become incorporated into the measurements. However, improved methods of filtering out the effects of long-term vertical land movements, as well as greater reliance on the longest tide-gauge records for estimating trends, have provided greater confidence that the volume of ocean water has indeed been increasing, causing the sea level to rise within the given range.

It is likely that much of the rise in sea level has been related to the concurrent rise in global temperature over the last 100 years. Within this time scale, the warming and consequent thermal expansion of the oceans may account for about 2 to 7 cm of the observed sea level rise, and the observed retreat of glaciers and ice caps may account for about 2 to 5 cm. Other factors are more difficult to quantify. The rate of observed rise of sea level suggests that there has been a net positive contribution from the huge ice sheets of Greenland and Antarctica, but observations of these ice sheets do not yet allow meaningful quantitative estimates of their separate contributions. The ice sheets remain a major source of uncertainty when accounting for past changes in sea level due to insufficient data about these ice sheets over the last 100 years.

15.2.2.3 Polar Ice Melting

Melting glaciers and land-based ice sheets also contribute to rising sea levels, lowering sea temperature and salinity and threatening low-lying areas around the globe with beach erosion, coastal flooding, and contamination of freshwater supplies. Each year, a volume of water equivalent to the upper 7 mm of all of our planet's oceans falls as snow on the Antarctic ice sheet, and a roughly equivalent amount of ice slips into the sea via glaciers. However, the ice sheet is seldom in a state of balance. From one Ice Age to the next, and from one season to the next, the amount of snow falling differs from the amount of ice leaving, causing parts of Antarctica's frozen reservoir to be alternately drained and replenished. When the ice sheet grows, the global sea levels fall; when it shrinks, sea levels rise. Sea-ice draft is the thickness of the part of the ice submerged under the sea. Sea-ice data acquired in the Arctic pole by submarine researchers have shown a similar melting trend between the periods 1958-1976 and 1993-1997: the mean ice draft at the end of the melt season has decreased by about 1.3 m in most of the deep water of the Arctic Ocean, from 3.1 m during 1958-1976 to 1.8 m during the 1990s. The ice draft is therefore more than a meter thinner than two to four decades earlier (Fig. 15.6). The main draft has decreased more than 3 m under 2 m, and its volume has decreased by some 40%. Given that Antarctica is exposed to the atmosphere and oceans, sudden changes in the Earth's climate can also alter this balance. Because Antarctica is a largely frozen environment, however, its ice was expected

Fig. 15.6 Increasing melt period since the end of the "Little Ice Period" to 2004 in the northern Atlantic, Greenland, Paleartic, and Neartic Regions (data are from the United Nations Environment Programme (UNEP))

to respond only slowly to the recent increase in global temperatures, which have climbed 10 times faster during the twentieth century than at any other time over the last 1,000 years. Now, however, satellite observations tell a different story. Coastal glaciers are retreating and thinning at various places around the entire continent, and it seems that the vast quantity of heat delivered by the Earth's gradually warming oceans may be to blame. The first cracks in Antarctica's icy armor appeared during the mid-1990s. Massive ice shelves floating in bays around the Antarctic Peninsula, a narrow mountain chain that extends northward toward South America, began to disintegrate.

15.2.2.4 Altitudinal Changes

One of the best places to look for changes in plant and animal life that may be caused by a climate change is in the mountains. As the globe warms up, mountaintops get warmer too. Trees start growing at higher altitudes than before, and the tree line shifts upward. The alpine timberline zone is highly sensitive to climate variability (Fig. 15.7). The rise of temperatures during the vegetation period over long periods induces also a rise of the tree line with higher forest stand density. Temperature reductions, however, lead to less dense forests and a drop of the timberline. The Alps records from the last 80-100 years show that plants have been migrating upward at a rate of about 4 m every decade. Researchers of the Institute for Geography at the University of Innsbruck have conducted dendrochronology analyses in the Tyrolean Central Alps where the timberline region is dominated by Swiss stone pine, demonstrating that the distribution area of adult trees and regeneration has moved upward during the last 150 years. In Nevada (USA) the Engleman spruce had moved its habitat upslope 650 feet in just 9 years. Organisms that live on mountains may face a grim future because mountaintops are in many ways like islands. They are isolated clearings that poke up above the tree line (Nicolussi and Patzelt 2006).

Loss of glacier volume has been more or less continuous since the nineteenth century, but it is not a simple adjustment to the end of an "anomalous" Little Ice Age. The data collected by Dyurgerov and Meier (2000) during the 1961-1997 period show trends that are highly variable with time as well as within regions; trends in the Arctic are consistent with global averages but are quantitatively smaller. The averaged annual volume loss is 147 mm year-1 in water equivalent, totalling 3.7 x 103 km3 over 37 years. The time series shows a shift during the mid-1970s, followed by more rapid loss of ice volume and further acceleration during the last decade; this is consistent with climatologic data. Perhaps most significant is an increase in annual accumulation along with a rise in melting; these changes produce a marked increase in the annual turnover or amplitude. The warming of air temperature suggested by the temperature sensitivities of glaciers in cold regions is somewhat greater than the global average temperature rise derived largely from low-altitude gauges, and the warming is accelerating (Dyurgerov and Meier 2000).

Courrent climate -Changing scenario

Courrent climate -Changing scenario

Fig. 15.7 Impact of climate on mountain vegetation belts. (Left) current vegetation. (Right) simulated vegetation belts under warming and drying climate
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