The Physics of Energy, page 154
The predicted distribution of temperatures across the planet in the RPC scenarios is shown in Figure 35.17. In each of the four scenarios, warming is greater in the interior of large continental land masses than over the ocean, and the maximum warming occurs at high latitudes. The bulk of the Antarctic ice sheet remains intact, which along with the strong Antarctic circumpolar current keeps the southern high latitudes from warming as much as the northern high latitudes. In all scenarios the Arctic region experiences the greatest local warming, with temperature increases of more than twice those of equatorial regions.
Figure 35.17 Distribution of local temperature changes in the four RCP scenarios [245].
One way to characterize the temperature increase due to global warming is to relate the resulting local temporary climate situation to earlier times with similar global surface temperatures. As described above and depicted in Figure 35.13, Earth’s climate has been gradually cooling for the last 50 million years. As the temperature rises due to CO2-induced radiative forcing, for parts of the planet the climate effect may be similar to moving backward in time toward the Eocene epoch, with a greater increase in temperature corresponding to an earlier time period in Earth’s history. If the average global temperature were to rise by 2100 by on the order of 3°C, the most recent corresponding time in Earth’s history would be over 3 million years ago, in the mid-Pliocene. At that time, atmospheric CO2 levels are believed to have been around 360–400 ppm, mean global surface temperatures were 2–3°C above current values, and sea levels are believed to have been roughly 25 m higher than the present day. As in the predictive climate model results, temperatures at high latitudes were significantly higher, though tropical temperatures may have been only slightly warmer than current values. The analogy to earlier times is, however, imperfect, since many parts of the climate system – most notably the cryosphere – would not have time to reach a new equilibrium, so sea levels presumably would not rise nearly as high as in the Pliocene before atmospheric carbon levels would have dropped again. Of course, however, organisms would have no time to adapt through evolution to the changed climate so there would be very strong impacts on many ecosystems. Raising temperatures beyond 3°C would run the clock further backward in time towards the Eocene, when global temperatures are believed to have been more than 7°C above current values.
At mid-latitudes, the change in temperature will be most notable at the seasonal extremes. Winters will be warmer and shorter, summers will be longer and hotter. A simple way of roughly characterizing the effect, stated clearly by Ruddiman [257], is that a doubling of CO2 would likely lead to warming at mid-latitudes that might cause a shift in usual seasonal conditions by roughly one month. So with a doubling of atmospheric CO2 levels by 2100, at mid-latitudes in the Northern Hemisphere, spring would come one month earlier, May would be like June of 100 years earlier, etc. Only in the middle month of summer would temperatures tend to reach unfamiliar regimes. Of course, however, the effects of warming will impact different local regions in very different ways, and will change the distribution of precipitation and extreme weather events, as we discuss in further detail below in §35.3.4.
35.3.2 Arctic Climate
The Arctic region is likely to be the area of the planet most affected by climate change. While global average temperatures did not increase significantly in the decade from 2000–2010, the extent of sea ice in the Arctic ocean diminished substantially during this period and has continued to decrease on average as the temperature has risen since 2010. The extent of sea ice reached the lowest level ever recorded in 2012 (see Figure 35.20(a)), going below 4 M km compared to an average minimum of 7 M km over the 20 years from 1979–2000. The rate of summer Arctic ice melt is increased not only by radiative forcing from atmospheric CO2, but also through ice albedo feedback, black soot, increased current and wind patterns, and weaker ice as the ice sheets diminish in size and smaller regions are covered with multi-year ice that has survived the summer thaw from previous years. Submarine and satellite measurements of ice thickness over the last 25 years have shown a steady decrease in ice thickness (see Figure 35.20(b)).
Figure 35.18 With a doubling of atmospheric CO2 and a temperature rise of 2–3°C by 2100, terrestrial conditions could roughly correspond to those of the mid-Pliocene over 3 million years ago, though the analogy is imperfect as ecosystems and many components of the climate will not have time to adapt to a new equilibrium. (Credit: Mauricio Anton)
Figure 35.19 A map of the extent of Arctic sea ice from September 2012 compared with the long-term average (red). (Credit: The National Snow and Ice Data Center (NSIDC), University of Colorado)
Figure 35.20 Trends in extent and thickness of Arctic sea ice. (a) Arctic sea ice extent in the month of September from 1978 through 2016. The red dashed line shows the trend. (b) Spring and autumn ice average thickness measured by submarine and satellite. Shaded bars indicate quality of data. (Credit: (a) NSIDC; (b) R. Kwok and D. A. Rothrock, Decline in Arctic sea ice thickness from submarine and ICEsat records: 1958–2008, Geophysical Research Letters, 36, L15501 (2009))
In addition to the positive ice albedo feedback, decreased Arctic sea ice exposes more ocean surface directly to the atmosphere. Increased melting and evaporation increases the water vapor content in the air, increasing the net (positive) water vapor plus lapse rate feedback. As the ice cap shrinks, heat is also released from the ocean to the atmosphere more rapidly, leading to more melting and warmer temperatures in the late summer and fall, and providing another positive feedback. In recent years the rate of Arctic ice melt has been significantly faster than had been predicted by most models and by the IPCC 2007 report. The models used in the 2013 report come closer to matching observed data but still underestimate the rate of Arctic ice melt. Roughly half of the RCP 4.5 models project that the Arctic will be essentially free of summer sea ice by 2100, and some recent estimates predict that this may occur substantially sooner, with predicted dates of ice extent below 1 million km ranging from 2020 to 2050.
35.3.3 Rising Seas and Melting Ice
Rising temperatures lead to rising sea levels for two main reasons: thermal expansion and ice melt. The density of salt water decreases with temperature, as does the density of fresh water above about 4°C (see Figure 35.21). This behavior is typical of most fluids, since increased molecular activity increases the mean distance between molecules. Therefore the volume of seawater expands as it warms. (When salt water freezes, the salt content of the ice is much reduced; ice floats on both salt and fresh water, a fact that has profound consequences for climate and life on Earth.) Global sea levels rose by about 20 cm between 1900 and 2000 (see Figure 35.22). Over half of this sea level rise has been due to thermal expansion. A simple estimate (see Box 35.4) shows that warming a 200 m vertical column of water by 1°C leads to a rise in sea level of roughly 4 cm, which suggests that thermal expansion is unlikely to lead to more than 10–20 cm of sea level rise in the next century. This conclusion is compatible with the RPC simulation results shown in Figure 35.15. The contribution of thermal expansion and other factors to sea level rise over the next century in the different RPC scenarios is broken down in Figure 35.23; in all these scenarios, thermal expansion contributes the largest component, between roughly one third and one half, of projected sea level rise.
Figure 35.21 The (a) density (b) derivative of density with respect to temperature for fresh and seawater. Note that seawater is more dense than fresh water below 50°C but that its density decreases more rapidly with temperature. Note also that the density of fresh water actually increases with temperature between 0°C and 4°C, which is why ice floats on fresh water.
Figure 35.22 Global mean sea level rise since 1880. (Credit: US Environmental Protection Agency)
Box 35.4 Sea Level Rise
We can get a rough estimate of the impact of thermal expansion on sea levels by considering the expansion of the oceanic mixed layer in the tropics. The coefficient of thermal expansion , defined by
determines the amount by which water expands with temperature, and a simple calculation relates to the density ρ,
For seawater near 15°C, K (see Figure 35.21). For simplicity we treat this coefficient as constant. If the temperature increases by 1°C, then a mixed layer of depth 200 m will expand vertically by cm.
If the oceanic mixed layer warms by 3.2°C over a period of 30 years (as in Problem 35.6), then a total rise of ~14 cm is expected. Although the temperature increase in the twentieth century has been less than 1°C, a rise in deep ocean temperatures as well as the surface mixed layer has increased the effects of thermal expansion beyond our simple estimate.
Figure 35.23 Projected contributions of different factors to sea level rise over the coming century in four RCP scenarios [245].
The other primary source of rising sea levels is the melting of ice over land. Melting of sea ice, such as much of the Northern polar ice cap, does not affect sea levels since the ice is floating on the ocean surface, displacing almost exactly the volume that it will fill after melting. Melting of ice over land, however, has led to substantial changes in sea level over geological time scales. As mentioned above, it is estimated that sea levels have risen about 120 m since the last glacial maximum 20 000 years ago. And in the much warmer climates of the Cretaceous and Eocene, when there were no continental ice sheets, sea levels are believed to have been 100–200 m above current levels. Currently the Antarctic ice sheets contain the greatest mass of ice on land, and there are substantial quantities of land ice over Greenland and in mountain glaciers at high altitudes around the world. As temperatures warm, each of these masses of ice will react to the changing climate. Much of the rise in sea levels over the last century that was not due to thermal expansion came from melting glaciers, not including the primary Greenland and Antarctica ice sheets (an ice sheet is generally defined as a glacial mass of ice over land having area above 50 000 km). In the RCP projections (Figure 35.23), glacial melting is expected to constitute the next-largest contribution to sea level rise over the next century, following thermal expansion. In these projections, 15% to 85% of the volume of glaciers outside Antarctica are expected to be lost by 2100.
The energy needed to melt continental ice sheets is tremendous, and to complete such a transformation takes many thousands of years. A simple back-of-the-envelope calculation gives a sense of the rate at which energy used to melt ice can lead to sea level rise. If we assume that 1% of the excess energy from a radiative forcing of 3.7 W/m went to melt ice, then a short calculation (see Problem 35.8) shows that sea levels would rise roughly 5 cm/decade. It is useful to consider separately the various possible contributors to substantial sea level rise through ice melt and related changes in the cryosphere.
Antarctica Antarctica is covered by ice sheets several kilometers thick over an area of roughly 14 M km. The total volume of ice is roughly 30 M km, enough to raise global sea levels by well over 50 m. In East Antarctica, the ice sheets lie on ground above sea level. The West Antarctic ice sheet rests largely on the sea bottom, with edges that extend into large floating ice shelves. Most of the ice mass added in precipitation in Antarctica is offset by loss of ice mass to the ocean, with only a small fraction directly melting. As temperatures in the Southern Hemisphere rise, Antarctica receives increased precipitation. In East Antarctica, this may be enough to keep total ice volume roughly constant, or even to increase ice volume in the short term. In West Antarctica, melting increases the rate of flow of ice sheets into the ocean, and currently the rate of ice loss exceeds the accumulation rate. The Scientific Committee on Antarctic Research (SCAR) combines ongoing scientific research results into regular publications that document the current scientific understanding of climate change to Antarctica. In their 2016 update [271, 272], they summarize several studies that suggest that with an increase in temperature of roughly 2°C in the Southern Ocean, there is a substantial risk of a West Antarctic ice sheet collapse over the next thousand years, and that high emissions scenarios could lead to ice loss giving a sea level rise of 1–3 m by 2300. A large-scale collapse of the West Antarctic ice sheet could lead to a rapid rise in sea levels by 5–6 m. Current models do not suggest that the conditions leading to such an event will be reached in the next century, but these models may not cover the range of deformation in the ice sheet that could be reached with rapid ice shelf collapse, and observations made to date do not rule out a significant tail in the probability distribution that would include several meters of sea level rise on a century time scale. A better understanding is needed for a high-confidence projection.
Greenland Greenland has a total ice volume of roughly 2.85 M km, less than 10% of the ice volume of Antarctica. Because the Arctic region is expected to experience greater warming than the southern polar region, however, it is expected that a larger fraction of Greenland’s ice sheet will melt in the coming centuries. The rate of ice melt in Greenland is accelerated by additional anthropogenic effects, particularly black soot (§34.4.4) from fossil fuel emissions that absorbs incoming sunlight at a high rate. Melted water on the surface passes rapidly down through channels in the glacier to the ground beneath, where it has the effect of lubricating the system and increasing the rate of flow of the glacier, further increasing the melt rate. Recent estimates indicate that the rate of ice melt in Greenland rose from roughly 55 Gt/y in the 1990s to roughly 290 Gt/y in 2011. In the IPCC RCP scenarios, the contribution of Greenland ice melt to sea level rise grows faster than linearly, and by 2500 contributes at a comparable rate to thermal expansion. If Greenland’s ice sheet were to melt completely, it would raise global sea levels by roughly 7 m. In any scenario currently contemplated, however, this would take many centuries, and is deemed unlikely to occur unless temperature increase goes beyond a threshold of around 4°C.
Mountain glaciers Most of the world’s mountain glaciers are small enough to be significantly affected by global warming over a period of decades. Glaciers form a relatively easy to measure signal of climate change; during warmer periods, glaciers retreat, during cooler periods they advance. Following the Little Ice Age, glaciers retreated from about 1850–1950. They advanced again during the slight cooling from 1950 to 1980, and almost all of the world’s glaciers have been retreating steadily during the warming trend since 1980. Many glaciers, such as those in the Himalayas and elsewhere in the Northern Hemisphere, are experiencing accelerated melting due to black soot particulates. If temperatures rise by 3°C or more, as is predicted with a doubling of pre-industrial CO2 levels, it is likely that many of the world’s glaciers will be gone by the end of the century. This contributes to a significant fraction of the sea level rise predicted by the IPCC. It has been estimated that a complete melting of all alpine glaciers would lead to a sea level rise of roughly 0.5 m. Since glaciers are a primary source for fresh water in many parts of the world, the melting of glaciers will also compromise water supplies in many countries.
Incorporating all factors, the IPCC 2013 report projected that total sea level rise in the twenty-first century would likely be between 40 cm and 100 cm, depending upon the scenario. The largest uncertainty in these projections is the possibility of an unexpectedly large contribution from rapid dynamics of the West Antarctic ice sheet. A rise in sea level of 1 m would cover roughly 1 M km of coastal land area [273]. While this is less than 1% of the 150 M km of total terrestrial land area, it would include many sensitive ecosystems and population centers. A rise in sea level of 6 m would cover a total of roughly 2.2 M km of land; note that this land area is comparable to the desert area computed in §22 that would be needed to supply all current and future world energy needs at 3–6% solar conversion efficiency.
35.3.4 Changes in Weather
As the climate warms, many changes may occur in local weather patterns. Such local effects are difficult to predict with accuracy. A number of global trends, however, are easier to understand and predict. As mentioned above, there will be more warming over land than over oceans, and the Arctic region will experience the greatest warming due to a combination of increased Northern heat transport, ice albedo feedback, and increased ice melt from particulates and changing weather and current patterns.
The increased greenhouse effect leads to an increase in downward IR radiation flux from the atmosphere as a fraction of total incoming radiation. Since the time scale for the atmosphere to reach thermal equilibrium is measured in hours or days, this will even out diurnal fluctuations and increase daily minimum temperatures more than daily maximum temperatures. Due to the seasonal shift described above, there will be more very hot days and heat waves and fewer very cold episodes during the seasonal extremes.
Increased flow of energy through the ocean surface layer and increasing temperatures over land will lead to an overall increase in evaporation, leading in turn to a net increase in precipitation. Many desert and arid land areas exposed to greater warming will become drier as the increase in evaporation exceeds the increase in precipitation. Warming will cause precipitation intensity to increase in other areas but occur with reduced frequency, with greater energy in the system producing more extreme weather events.
35.3.5 Ecosystem Impact
Impact on ecosystems goes somewhat outside the purview of this text. A few observations, however, are relevant and help to complete the picture of the possible effects of future warming.
