Track 1) Global climate change and its impacts on natural systems and people

Earth’s climate changes on many different time scales, ranging from tens of millions of years to decadal and even shorter time scale variations. In the last 2.5 billion years, several periods of glaciation have been identified, separated by periods of mild climate similar to that of today. Other periods are marked by global hot house type conditions, when the Earth had a very hot and wet climate, approaching that of Venus. These dramatic climate changes are caused by a number of different factors that exert their influence on different time scales. One of the variables is the amount of incoming solar radiation, and this changes in response to several astronomical effects such as orbital tilt, eccentricity and wobble. Changes in the incoming solar radiation in response to changes in orbital variations produce cyclical variations known as Milankovitch Cycles. Another variable is the amount of heat that is retained by the atmosphere and ocean, or the balance between the incoming and outgoing heat. A third variable is the distribution of landmasses on the planet. Shifting continents can influence the patterns of ocean circulation and heat distribution, and placing a large continent on one of the poles can cause ice to build up on that continent, increasing the amount of heat reflected back to space and lowering global temperatures in a positive feedback mechanism.

Shorter term climate variations include those that operate on periods of thousands of years, and shorter, less regular decadal scale variations. Both of these relatively short-period variations are of most concern to humans, and considerable effort is being extended to understand their causes, and to estimate the consequences of the current climate changes the planet is experiencing. Great research efforts are being expended to understand the climate history of the last million years, and to help predict the future.

Cycles of CO2 and temperature showing 100,000, and shorter term cycles.

Variations in formation and circulation of ocean water may cause some of the thousands of years to decadal scale variations in climate. Cold water forms in the Arctic and Weddell Seas. This cold salty water is denser than other water in the ocean, so it sinks to the bottom and gets ponded behind seafloor topographic ridges, periodically spilling over into other parts of the oceans. The formation and redistribution of North Atlantic Cold Bottom Water accounts for about 30 percent of the solar energy budget input to the Arctic Ocean every year. Eventually, this cold bottom water works its way to the Indian and Pacific Oceans where it upwells, gets heated, and returns to the North Atlantic. This cycle of water circulation on the globe is known as thermohaline circulation. Recent research on the thermohaline circulation system has shown a correlation between changes in this system and climate change. Presently, the age of bottom water in the equatorial Pacific is 1,600 years, and in the Atlantic it is 350 years. Glacial stages in the North Atlantic have been correlated with the presence of older cold bottom waters, approximately twice the age of the water today. This suggests that the thermohaline circulation system was only half as effective at recycling water during recent glacial stages, with less cold bottom water being produced during the glacial periods. These changes in production of cold bottom water may in turn be driven by changes in the North American ice sheet, perhaps itself driven by 23,000 year orbital (Milankovitch) cycles. It is thought that a growth in the ice sheet would cause the polar front to shift southward, decreasing the inflow of cold saline surface water into the system required for efficient thermohaline circulation. Several periods of glaciation in the past 14,500 years (known as the Dryas) are thought to have been caused by sudden, even catastrophic injections of glacial meltwater into the North Atlantic, that would decrease the salinity and hence density of the surface water. This in turn would prohibit the surface water from sinking to the deep ocean, inducing another glacial interval.

Shorter-term decadal variations in climate in the past million years is indicated by so-called Heinrich Events, defined as specific intervals in the sedimentary record showing ice-rafted debris in the North Atlantic. These periods of exceptionally large iceberg discharges reflect decadal scale sea surface and atmospheric cooling are related to thickening of the North American ice sheet followed by ice stream surges associated with the discharge of the icebergs. These events flood the surface waters with low-salinity fresh water, leading to a decrease in flux to the cold bottom waters, and hence a short period global cooling.

Changes in the thermohaline circulation rigor have also been related to other global climate changes. Droughts in the Sahel and elsewhere are correlated with periods of ineffective or reduced thermohaline circulation, because this reduces the amount of water drawn into the North Atlantic, in turn cooling surface waters and reducing the amount of evaporation. Reduced thermohaline circulation also reduces the amount of water that upwells in the equatorial regions, in turn decreasing the amount of moisture transferred to the atmosphere, reducing precipitation at high latitudes.

Atmospheric levels of greenhouse gases such as CO2 and atmospheric temperatures show a correlation to variations in the thermohaline circulation patterns and production of cold bottom waters. CO2 is dissolved in warm surface water and transported to cold surface water, which acts as a sink for the CO2. During times of decreased flow from cold, high-latitude surface water to the deep ocean reservoir, CO2 can build up in the cold polar waters, removing it from the atmosphere and decreasing global temperatures. In contrasts, when the thermohaline circulation is vigorous, cold oxygen-rich surface waters downwell, and dissolve buried CO2 and even carbonates, releasing this CO2 to the atmosphere and increasing global temperatures.

Global warming pattern, showing stronger effects at high latitudes than equatorial latitudes.

The present day ice sheet in Antarctica grew in the Middle Miocene, related to active thermohaline circulation that caused prolific upwelling of warm water that put more moisture in the atmosphere, falling as snow on the cold southern continent. The growth of the southern ice sheet increased the global atmospheric temperature gradients, which in turn increased the desertification of mid-latitude continental regions. The increased temperature gradient also induced stronger oceanic circulation including upwelling, and removal of CO2 from the atmosphere, lowering global temperatures, and bringing on late Neogene glaciations.

Major volcanic eruptions inject huge amounts of dust into the troposphere and stratosphere, where it may remain for several years, reducing incoming solar radiation and resulting in short-term global cooling. For instance, the eruption of Tambora volcano in Indonesia in 1815 resulted in global cooling and the year without a summer in Europe. The location of the eruption is important, as equatorial eruptions may result in global cooling, whereas high-latitude eruptions may only cool one hemisphere.

It is clear that human activities are changing the global climate, primarily through the introduction of greenhouse gases such as CO2 into the atmosphere, while cutting down tropical rain forests that act as sinks for the CO2 and put oxygen back into the atmosphere. The time scale of observation of these human, also called anthropogenic, changes is short but the effect is clear, with a nearly one degree change in global temperature measured for the past few decades. The increase in temperature will lead to more water vapor in the atmosphere, and since water vapor is also a greenhouse gas, this will lead to a further increase in temperature. Many computer-based climate models are attempting to predict how much global temperatures will rise as a consequence of our anthropogenic influences, and what effects this temperature rise will have on melting of the ice sheets (which could be catastrophic), sea level rise (perhaps tens of meters or more), and runaway greenhouse temperature rise (which is possible).

Climate changes are difficult to measure, partly because the instrumental and observational records go back only a couple of hundred years in Europe. From these records, global temperatures have risen by about 1 degree since 1890, most notably between 1890-1940, and again since 1970. This variation however, is small compared to some of the other variations induced by natural causes, and some scientists argue that it is difficult to separate anthropogenic effects from the background natural variations. Rainfall patterns have also changed in the past 50 years, with declining rainfall totals over low latitudes in the Northern Hemisphere, especially in the Sahel, which has experienced major droughts and famine. However, high-latitude precipitation has increased in the same time period. These patterns all relate to a general warming and shifting of the global climate zones to the north.