Geography 40
Global Environmental Change
Fall 2002

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Lecture 36: Global Warming and Future Change Reading: Ruddiman, chapter 19, on reserve in Earth Science Library A theme to keep in mind when discussing Predictions of climate change related to Global Warming is that, while we are certain that Greenhouse gases (CO2 is the focus of discussion, but there are other gases involved, like CH4 – Methane) absorb and reradiate long-wave radiation, thereby warming the Earth’s surface, there is a level of uncertainty involved in predicting what climate changes we will see in the near future, and in the next few hundred years. The level of certainty depends on: 1) ability to predict the total amount and rate of carbon emissions and 2) ability to understand all the complexities of the Earth System mannetal Temperature records spanning the last 1000 years indicate that a general, slight cooling trend prevailed for much of the last 1000 years. In the last 150 years, however, that trend has reversied and an unprecedented high rate of temperature increase has been observed. 0.6C rise over approximately 100 years. This is a rate that is far higher than can be explained by any of the natural cycles of climate change we have discussed in class (e.g., natural changes in Greenhouse gases which change on timescales of many millions of years, orbital cycles which act on the scale of many thousands of years, or other shorter-term climate fluctuations related to sunspot activity). Ruddiman Greenhouse and Natural Changes: the magnitude of warming caused by increased greenhouse gases should overwhelm the effects of natural climate changes in the next several hundred years. Uncertainties involved in predicting Climate impacts from human-caused Greenhouse warming include estimating future increases in greenhouse gases. This centers on 2 main issues: 1) how much CO2 will humans emit; and 2) how will the Earth’s system distribute the additional CO2 among the atmospheric, oceanic and terrestrial reservoirs? Projected Carbon Emissions Projections are based on 3 factors: % increase % increase % change in changes in In carbon = in population X emissions X efficiency of Emissions per person carbon use Ruddiman Projections vary with uncertainties in future populations, living standards, and conservation efforts and technology. 2xCO2 scenario is optimistic, 4xCO2 is also possible. The two projections encompass much of the range of IPCC estimates. Implicit in the upper curve (4xCO2) is the assumption of minimal efforts to curb emissions, with economic benefit continuing to be the major basis of decision making. This trend shows emission rates peaking at a value of 3 to 4 times the modern level just before 2200 and then falling back below present-day values near the year 2300. Ruddiman Predicting the future path of CO2 concentrations in the atmosphere is more difficult than estimating emission levels. In addition to uncertainties about emissions, scientists face the question of how the climate system will redistribute the pulse of excess CO2 among the carbon reservoirs. An assumption can be made that the atmosphere will continue to receive just over 1/2 total carbon emissions with rest going to the ocean and the biosphere. Over longer timescales, other considerations come into play. Within a few hundred years, most of the pulse of excess CO2 added to the atmosphere by humans will be mixed into the subsurface ocean, making ocean water more acidic. Over the course of several thousand years , the acidity produced by this extra CO2 will cause some of the CaCO3 on the sea floor to dissolve. The processes that will initially mix the excess CO2 into the deeper ocean are not fully understood. Vegetation also plays a role in absorbing excess CO2. Ability of plants to take advantage of the extra CO2 available for growth, as well as the possibility of expanding into formerly frozen areas will be countered by 2 factors: first, humans continue to reduce area of forests; second, amount of carbon stored in vegetation may also decline as some kinds of vegetation are forced out of areas before others can fully replace them Ruddiman Projected range of future temperatures is between 2.5 and 5 degrees C, using the two scenarios of doubled, and 4x CO2 emissions. Note that the peaks of temperature are different from that of CO2 concentrations in the atmosphere. The lower projection shows a 2.5C warming by 2200, more than 4X the warming of .6C seen in the 20th century. The upper projection shows a 5C warming, almost 10 times the size of warming seen. The sensitivity of 2.5C to a doubled CO2 concentration could be wrong by a factor of 50% in either direction. Lower sensitivity of 1.5C would mean that Earth’s temperature would warm more slowly and reach a smaller peak in the mid 23rd century. Ruddiman Average global temperatures in the next 200 years is projected to reach levels comparable to those of many millions of years ago. But there is no analog to the world that will exist - one with high temperatures, but containing ice sheets. The temperatures changes will occur too rapidly for ice sheets to respond. Impacts of Global Warming echam_sealevel Apart from rising temperatures, the next obvious impact will be higher global sea levels. These model curves go back to the turn of the 20th century, and you can see sea level has already risen more than 10 centimeters. Much of this rise is due to Thermal Expansion of the oceans (ocean water expands somewhat with a rise in temperature). Ruddiman While the interior ice sheets may be slow to respond to changes in climate, smaller ice shelves are sensitive to temperature change. One of the smaller ice shelves in Antarctica has been receding recently after growing over a 400-year cooling period. Evidence from sediment and glacial core samples indicates there have been other periods of warmth in Antarctica, but the present temperature is the warmest in 500 years and increasing rapidly. Ice from the interior of Antarctica flows in ice streams to the shelves along the margin. In a warmer world, ice shelves may be vulnerable to destruction by rising ocean temperatures, which may in turn accelerate flow in the ice streams. This could lead to a "Sudden Surprise" impact, as a pulse of melt water could lead to a sudden high rate of sea level rise. We saw that this happened around 14,000 years ago, and has been blamed on the collapse of this part of the Antarctic Ice Sheet. Arctic Ocean Meltdown Sea ice in the Arctic Circle is decreasing at a rate of 37,000 km2 per year and is now about 40% thinner than it was 4 decades ago. this sea ice recession has major implications, from loss of polar bear habitat and increased precipitation over parts of the Arctic, to a theorized influx of fresh water into the North Atlantic that could cut off the ocean "conveyor belt" that transports heat around the globe and helps govern the Earth's climate surfairchange A collapse of thermohaline circulation due to melting of Arctic sea ice could, ironically, result in cooling of northern Europe. We discussed a similar scenario which apparently happened 11,000 years ago, the Younger Dryas. Other Impacts of Global Warming Changes in the depth of mountain snow packs and glaciers, and changes in their seasonal melting, can have powerful impacts on areas that rely on freshwater runoff from mountains. Rising temperatures may cause snow to melt earlier and faster in the spring, shifting the timing and distribution of runoff. These changes could affect the availability of freshwater for natural systems and human uses, such as agriculture. If freshwater runoff is reduced in the summer months because of earlier melting, soils and vegetation may become drier, increasing the risk and intensity of wildfires. Changes in stream flow and higher water temperatures also could affect insects and other invertebrates that live in streams and rivers, with repercussions up the food chain for fish, amphibians, and waterfowl. Next time – Guest lecturer on Global Warming and the spread of Infectious Diseases…