I am reading Chapter 20 this week. This is the last chapter in the book.

All notes were things I highlighted in my book.

Chapter 20: Future Climate Change

20-1 Factors Affecting Future Carbon Emissions

• Atmospheric CO2 concentrations have been rising at rates of 2 ppm/year
• For as long as fossil fuels remain reasonably abundant, future carbon emissions can be approximated by multiplying three factors: increase in carbon emissions = increase in population x change in emissions per person x changes in efficiency of carbon use
• This equation could also be expressed as: population growth x economic growth x technology.
• Attempts to project future population increases are complicated by the tendency of birth rates to fall as per capita income rises and by centralized efforts to slow population growth in China.
  • Global population is projected to rise rapidly from 7 billion in the year 2010 to 9 or 10 billion by 2050, but with a leveling off of the trend after mid-century.
  • As developing countries reach higher levels of wealth and parents in those countries choose to have fewer children. This trend has already been underway for decades in fully industrialized countries.
•  The change in carbon emissions per person is primarily linked to the average standard of living.
•  The average efficiency of carbon use—is by far the most difficult to constrain because of uncertainties about several factors, including the mix of carbon-based fuels.
  • We will see peaks in annual production of oil followed by a decline in annual production.
  • Natural gas will follow a similar path because more gas will become accessible to new recovery techniques.
•  Compared to coal, oil and gas are relatively “clean” fuels that emit smaller amounts of CO2 per unit of energy produced.
  • When oil and gas use declines, coal and other “dirty” sources will increasingly become the primary carbon-based fuels until the end of the fossil-fuel era.
  • bituminous coal produces far more CO2 per unit of usable energy.
  • Oil sands found in vast deposits under the western Canadian prairies are another potentially enormous contributor of carbon emissions.
• The best hope for reducing future carbon emissions lies in technology, a part of the “efficiency” term in the equation.
  • Promising areas include: solar and wind energy, biofuels, hydrogen powered vehicles, and nuclear power plants that do not produce CO2 or other greenhouse gases.
  • Future increases in the cost of fossil fuels will help to accelerate the technology-based search for alternative energy.

20-2 Projected Carbon Emissions and CO2 Concentrations

• The Intergovernmental Panel on Climate Change (IPCC) has predicted a wide range of possible carbon emission trends for the rest of this century.
• The higher projection is based on the pessimistic assumption that efforts to curb emissions will have little or no effect, because assessments of economic benefits will guide national and individual decisions about energy use.
• The lower projection assumes that individuals, civic groups, cities, and nations will take strong action to curb carbon emissions and that advances in technology aid this effort.
• The simplest assumption is that the atmosphere will continue to receive just over half of the total carbon emissions in the future, with the rest entering the ocean and the biosphere in the same proportions as today.
• The part of the CO2 emissions that will be taken up by the ocean in the future is difficult to predict.
  • Geochemists have gained some insight into this problem in recent decades by tracking the gradual penetration into the ocean of tracers produced by human activities.
• Climate scientists have found that continental vegetation during recent decades has absorbed extra CO2 because of increased fertilization of plant growth—the CO2 fertilization effect.
  • If this fertilization trend continues in the future, CO2 will continue to be taken up by growing vegetation, and by the expansion of trees into regions of Arctic tundra.
  • As the vegetation approaches or reaches its full natural response to the extra CO2, it no longer plays a significant CO2-fertilization role.
• Projections of future atmospheric CO2 concentrations depend on the complex interaction of anthropogenic emissions and its slow removal by the ocean and the biosphere
• The projected CO2 trends reach their peak values later than the projected CO2 emissions trends, and they remain at high levels for a much longer time.
  • The reason for this relative delay in timing is the slow rate of CO2 removal by the ocean.
  • It will take several centuries for the slowly circulating deep ocean to absorb most of the excess CO2 emitted by humans, and 10–15% of the fossil-fuel pulse will stay in the atmosphere for tens of thousands of years or more.
• As a result, new CO2 emitted to the atmosphere in the future will be added to older CO2 left from emissions in prior decades and centuries.
  • Even after CO2 emissions generated by humans begin to drop, the remaining emissions will continue to push the cumulative atmospheric concentration to higher levels. (Lag)
• Over time, the acidity produced by the CO2 absorbed in the deep ocean dissolves some of the CaCO3 on the seafloor and neutralizes CO2 in the ocean. The fate of our pulse of excess atmospheric CO2 will be a slow-acting chemistry experiment carried out in the deep ocean.
• Both of the projections ignore the “equivalent CO2” increases in other greenhouse gases that have amounted to almost half of the industrial era emissions to date.

20-3 World in Climatic Disequilibrium

• The high-CO2 world of the future will be a place of major climatic disequilibrium because the high-CO2 pulse will arrive too quickly for all parts of the climate system to come into full temperature equilibrium. Our future world will be characterized by a strange mix of faster-responding parts of the climate system like the atmosphere, land surfaces, vegetation, and upper ocean, and slower-responding components like the deep ocean and ice sheets.
• The faster responding parts will register the CO2-driven warming within just a few decades, the slower-responding deep ocean will react much more slowly, and the ice sheets on Greenland and Antarctica will not have fully reacted to the high-CO2 levels even by the time the pulse begins to decline.
• Near the slow-responding ice sheets, which strongly influence regional climates by their high albedo and their effect on atmospheric winds, the lingering cold caused by the ice will suppress part of the fast response of the atmosphere and the nearby surface ocean to the higher CO2 levels.
• Farther from the ice, the faster-responding parts across most of the climate system will react strongly to the new warmth. Because this kind of disequilibrium has not occurred in Earth’s past, we have no perfect analogs for future climate.

20-4 Fast Climate Response in a 3 x CO2 World

• In a 3 x CO2 world, north polar regions will warm enough to eliminate shallow permafrost, tundra, all summer sea ice and probably most winter sea ice.
• If these trends continue, the Arctic Ocean will be completely ice-free in summer within a few decades, long before the peak 3 x CO2 concentration is reached.
  • With little or no sea ice through the warmer part of the year the winter Arctic will be much warmer than it is now.
•  In the Antarctic, where most sea ice disappears during modern summers except in a narrow strip along the coast of the continent
  • ice-free in summer and winter sea ice is difficult to predict, but it will likely be less extensive than it is now.
•  The north polar continents will be transformed by forests moving northward into areas that are now tundra.
  • Tundra will likely disappear in a 3 x CO2 world, replaced by evergreen forests extending to the Arctic coast.
• At present, near-surface and subsurface permafrost is widely distributed in the Arctic.
• In recent years, the discontinuous permafrost belt has begun to melt and will likely disappear in a 3 x CO2 world. The much colder band of continuous permafrost belt farther north remains unthawed at present.
• Almost every mountain glacier on Earth has already been retreating for a century or more, and the rates are accelerating. At the prevailing lapse rate of 6.5C per km, a 5C warming in the future should cause a vertical retreat of glaciers up the sides of mountains by more than 800 m, with larger changes at higher latitudes because of amplified warming.
•  Coupled models of climate and vegetation simulate a temperature-driven northward shift of vegetation.
•  Fauna and flora on mountainsides will also be affected by future warming. In order to remain in an optimal temperature regime, both will have to shift to higher elevations, and the transition will be easier for relatively mobile life-forms than less mobile ones.
•  Higher levels of CO2 in the atmosphere allowed C3 vegetation to survive in arid regions, but the gradual CO2 lowering during the last 10 million years made C3 vegetation less competitive with C4 grasses.
  • In the next two centuries, we will pass through the same 3 x CO2 threshold but heading in the opposite direction and at a much faster rate. As C3 shrubs and trees replace C4 vegetation, some arid and semiarid regions could become somewhat greener.
•  The warming during the next two or three centuries will also alter regional patterns of precipitation and evaporation in significant ways.
  • Evaporation will increase worldwide because warmer temperatures will permit air to hold more water vapor. With more water vapor in the atmosphere, global average precipitation will also increase, but in patterns that will vary from region to region.
  • With evaporation increasing, those areas that fail to receive more precipitation will become drier, while those that do receive more precipitation could become wetter. 
  • Unfortunately, because climate model simulations of regional precipitation often disagree, moisture trends are difficult to predict region by region.

20-5 Slow Climatic Reponses in a 3 x CO2 World

• The deep ocean is one of the slower-responding parts of the climate system, in part because of the 1,000 years or so it takes for an average molecule of water to circulate through the world ocean.
• During the 150 years of the industrial era, burning of fossil fuels and forests has added roughly 300 billion tons of carbon to the ocean, pushing the average pH of the ocean 0.1 unit lower equivalent to an alkalinity decrease of 30%.
• Several thousand billion tons of carbon remain buried in Earth’s carbon reservoirs and most of this carbon will end up in the ocean, intensifying the acidification.
• Oxygen in the ocean has decreased, partly because warm water holds less oxygen and warming of surface waters results in lower densities and less sinking of oxygen-rich water to great depths.
• By far the slowest-responding parts of the climate system are the ice sheets on Greenland and Antarctica.
• Most climate scientists think that much of the Greenland ice sheet will survive the pulse of high CO2 levels during the next few centuries and persist into the subsequent era of decreasing CO2 concentrations, but some disagree.
• The enormous ice sheet on eastern Antarctica was already very large by 13 million years ago. Today, most of this frigid ice sheet is starved for snow, with only a few centimeters per year falling across most of its high-elevation surface.
  • In a warmer 3 x CO2 world, the supply of snow should increase and allow faster annual accumulation of ice in the interior.
  • But this change toward positive mass balance in the ice sheet interior will be opposed by faster flow in marginal ice streams. Current estimates are that the marginal ice losses will outweigh the gains in the interior.
• This evidence suggests that the western Antarctic ice sheet will be considerably more vulnerable to melting in a 3 x CO2 world, but the amount of melting is uncertain.
• Projections of future sea level rise are very uncertain, mainly because of large disagreements about the amount of melting of Greenland and West Antarctic ice.
  • The rate of sea level rise for most of the 1900s averaged 1.7 mm/year, or a total of about 17 cm for the full century.
• The 2007 IPCC report projected that the rate of rise by the year 2100 would range between 1.8 and 5.9 mm/year, but most scientists now regard this as a considerable underestimate.
• Thermal expansion of a warming ocean is the largest factor in the 2007 IPCC estimate for the year 2100 at 1–4.1 mm/year.
• Over the rest of this century, sea level could rise by as much as half a meter or more, with further acceleration occurring during the 2100s.
• A sea level rises this large would impose major economic costs on many low-lying cities and on structures built along beach coasts. 

20-6 How will the Greenhouse World change human Life

• Consider the 4.5–5C global mean temperature increase in a 3 x CO2 world.
• Poleward amplification of this temperature change by albedo feedback from snow and sea ice retreat will boost the warming at upper to middle latitudes where many humans live to average increases of at least 7C and more in winter months.
• One way to evaluate these changes is in the context of modern seasonal changes.
• These changes will come on slowly over the next two or three centuries and will be masked by normal year-to-year variability.
  • Some of the most striking changes will occur in regions where snow and ice retreat northward.
• Warmer winters at high latitudes should extend the northern limit for growing many crops. Also, higher CO2 levels in the atmosphere should allow some plants to obtain the CO2 necessary for photosynthesis more quickly without exposure to the drying effects of evaporation.
• Unusually hot climate extremes will become common in a 3 x CO2 world. In the 1960s and 1970s, record highs and lows were more or less evenly balanced, but by the 2000s, record heat extremes outnumbered record cold by more than a factor of two.
  • Another way to assess the extreme climate trend is to map the frequency of local heat waves or cold waves that lie well outside the normal range of variability for each region.
• In the tropics and subtropics, where 80% of humans live, moisture balances will also be greatly altered.
• Areas that depend on runoff from meltwater from seasonal snows and shrinking glaciers for irrigation late in the growing season will also feel the effects of decreases in both sources of water.
• As for mountain glaciers, runoff from gradual melting in the modern world arrives in late summer, a very arid time of year in many regions. As mountain glaciers shrink and eventually disappear, this late-summer source of irrigation water will decrease and in most places come to an end.
• Because the warmer atmosphere takes up more water due to higher evaporation rates, precipitation will increase in some areas and on a global average basis.
• Future sea level rise caused by melting of land ice and expansion of ocean water will have an adverse effect on coastal populations.
• In heavily populated Bangladesh, a sea level rise within this range would displace tens of millions of people currently living less than one meter above current sea level. People in low-lying rural areas will have to retreat well inland as the sea invades.
• In built-up coastal regions such as the eastern and Gulf of Mexico coasts in the United States, ever-greater engineering efforts will become necessary and in fact have been underway for years in many regions.
  • Many of the world’s great cities will have to build massive walls for protection against a large rise in sea level.
• Some of the largest changes caused by future global warming may have relatively little broad economic impact on humans and yet still be crucial from an ecosystem perspective.

20-7 Methane Clathrate Release

• Methane exists as a gas in the atmosphere, but in Earth’s colder regions it also occurs in a frozen form known as methane clathrate, a mixture of methane and slushy ice.
  • Clathrates occur in deep-ocean sediments along continental margins, where the pressure produced by overlying water and sediments makes CH4 stable at temperatures well above freezing.
  • Clathrates also occur in the Arctic, both in shallow ocean sediments and in permafrost layers on land. The volume of CH4 stored in these reservoirs is enormous, far exceeding all carbon reservoirs in wetlands and livestock combined.
• Without large changes in climate, most methane clathrates will remain trapped in their present slushy deposits, but with the large future warming projected for north polar regions, could large amounts of this potent greenhouse gas be liberated?
  • Some scientists have expressed concern that if this were to happen, the emitted methane would further warm climate and set off a positive feedback loop that could drive far more global warming than our future CO2 emissions.
• Melting of the more vulnerable shallow permafrost and clathrates in less frigid regions will certainly release methane, but most scientists expect that the vast amounts of methane buried as clathrates at great depths in the most frigid regions of the high Arctic will remain largely unaffected.
  • Because permafrost thaws from the top down, the rates of penetration of atmospheric heat from anthropogenic global warming will be slow.
  • It would take many centuries for the highest surface temperatures caused by the future CO2 pulse to penetrate deep into permafrost and ocean sediments, and before that happens CO2 levels will be decreasing, and the planet will have begun to cool.
• In this view, methane clathrates will not cause a large-scale greenhouse surprise. But we may yet have much to learn about this matter.

20-8 Chilling of the North Atlantic and Europe

• One frequently mentioned possibility is that faster melting of Greenland ice could send enough freshwater to the North Atlantic Ocean to lower its salinity and slow or stop formation of deep water.
  • A relatively small drop in salinity in the Labrador Sea beginning in the 1970s lowered the density of the surface waters enough to prevent them from sinking during winters over the next two decades.
  • Large-scale melting of the Greenland ice sheet or increased precipitation over the North Atlantic caused by greenhouse warming could conceivably add enough low-salinity water to slow formation of deep water.
• A likely consequence of such a circulation change would be colder temperatures in northern Europe.
  • Today, the heat extracted from North Atlantic surface waters during formation of deep water is carried eastward into Europe and helps to keep its climate warmer in winter than Canada or Siberia at the same latitude.
  • Without this ocean heat, Europe would become colder than it is now. This scenario was the basis of the Hollywood movie The Day After Tomorrow, although it was carried to ridiculous extremes in the movie.
• Recent reassessments suggest, however, that a major cutoff of deep-water formation in the Atlantic is unlikely.
• The planetary wind system will continue to drive relatively warm and salty surface water northward from the tropical Atlantic Ocean, where relatively cold winter air masses will continue to extract heat.
• Partial reductions of deep-water formation could occur with minor lowering of salinity, but any cooling effect on Europe is now considered unlikely to offset more than a fraction of the larger greenhouse-gas warming in a 3 x CO2 world.

20-9 A different kind of anthropogenic climate surprise: Nuclear Cooling?

• One possible future climatic change does not depend on the greenhouse-gas emissions we will put in the atmosphere but instead depends on the dismal prospect of a future nuclear war.
• The primary result of a nuclear exchange would be firestorms caused by burning of fuel-rich urban and manufacturing areas that would send dark, carbon-rich smoke high into the atmosphere.
  • The soot reaching high into the stratosphere would linger for five years or more and absorb part of the Sun’s incoming radiation.
  • As a result, solar radiation reaching the surface would be reduced and temperatures would cool.
• Climate modelers have attempted to simulate nuclear exchanges on a variety of scales. Full nuclear exchanges between major nuclear powers would kill hundreds of millions of people immediately and would be catastrophic for even more of those left alive.
• Global temperatures would fall below those typical of the last glacial maximum 20,000 years ago and stay there for 5 to 10 years.
  • As a result, the growing season would shrink on every continent, disappearing entirely in the Ukraine and northernmost agricultural regions of North America. At the same time, global precipitation would fall by about half, further limiting agriculture.
  • The resulting food shortages would result in mass starvation that would multiply the level of mortality in regions far from the nuclear explosions and firestorms.
• An immediate disaster of this kind stands in stark contrast with the large, but slowly arriving consequences of anthropogenic global warming.
• Simulations indicate that even a more limited exchange between nations with smaller nuclear arsenals would kill tens of millions of people immediately and result in a cooling deeper than any level reached during the last thousand years.
• Model results suggest that global precipitation would fall by 10–20%, and the strength of the wet summer monsoons in Africa and Asia could be cut in half, further increasing human mortality in many regions.
• Early attempts during the 1970s to model nuclear exchanges produced effects so large that they were described as causing “nuclear winter.” Later models scaled back the size of the changes somewhat to a level called “nuclear autumn.”
• Recent models that incorporate a more complete range of interactive climatic processes and feedbacks have pushed the estimates back toward larger nuclear winter impacts. In any case, the climatic and other consequences of nuclear exchanges are too terrifying to be fully grasped.

20-10 Reducing greenhouse gas emissions to the atmosphere

• Limiting future emissions of greenhouse gases is a preemptive form of climate modification.
  • By not emitting greenhouse gases in the first place, or not as much as some current projections suggest, we can alter future climate.
• Compared to CO2, methane has a smaller net greenhouse effect, but it is of interest in the context of climate modification because its 10-year average residence time in the atmosphere is much shorter than that of carbon dioxide.
• Possible mechanisms for reducing methane emissions include trapping emissions from landfills more efficiently, reducing the amount of methane escaping from coal mines and natural gas drilling, building more efficient diesel engines, and minimizing emissions from rice paddies by altering irrigation patterns and kinds or amounts of fertilizer.
  • One idea to avoid adding huge amounts of CO2 to the atmosphere is to pipe excess industrial CO2 into the ocean or into old oil fields.
• As with many technological solutions, cost is a major issue. Many CO2-emitting sites lie far from the ocean or from oil reservoirs, and some of the CO2 would have to be piped hundreds to thousands of miles at considerable cost. Pumping CO2 into the ocean would obviously exacerbate the problem of acidifying the ocean.
• Another suggestion is to use genetic manipulation to develop microbes that feed on CO2 and turn it into some form of carbon other than greenhouse gases.
• Yet another suggestion focuses on CO2 already in the atmosphere. One way that CO2 leaves the atmosphere during glacial cycles is by planktic organisms fixing carbon in their shells and soft parts, dying, and sinking into the deep ocean.
  • The element iron is central to this mechanism, as first proposed by marine biologist John Martin.
• Although not very abundant in ocean waters, iron acts as a limiting nutrient on plankton growth. In many areas of the world ocean where major nutrients like phosphorous and nitrogen are readily available, production of planktic organisms would increase if more iron were available.
• More than a dozen ship-based experiments that have spread iron across patches of ocean water covering 1–10 km2 have shown a short-term increase in many kinds of plant plankton, and even blooms of a major group—diatoms.
  • While these results are promising, it is not yet clear how they would apply to slower long-term additions of iron over vastly larger areas.
• Using less energy through conservation measures is another way to reduce emissions. Several energy saving measures that are inexpensive or relatively inexpensive have increasingly come into use, but these alone can’t dramatically reduce atmospheric CO2 emissions.
• Conservation methods that work on a larger scale are more expensive and require support from local or national governments. The higher the costs of action rise, the more vocal the opposition from citizens and politicians becomes.

20-11 Reducing the effects of the Sun’s Heating

• Future aerosol additions to the atmosphere from human activities are difficult to predict. 
• Humankind faces the interesting dilemma: If all current emissions of industrial sulfates to the lower layers of the atmosphere were suddenly eliminated, precipitation would wash out most of the aerosol load within a few weeks, eliminating their cooling effect.
  • As a result, attempting to clean up our current sulfate aerosol emissions would cause a large and rapid additional warming.
• In a sense, we have already run a small-scale version of this experiment.
  • After the massive industrialization during World War II, sulfate aerosol loads in the atmosphere increased very rapidly, and the warming trend that had been underway for several decades slowed or was even reversed between the 1940s and 1970s.
  • This small net cooling is generally attributed to increased aerosol levels in the atmosphere countering or even outweighing the effects of rising greenhouse-gas concentrations.
  • Then, in the late 1970s, laws to reduce smokestack emissions began to be enacted, aerosol concentrations in the atmosphere fell, and global temperature began a rapid rise.
  • This timing suggests that cleaning the air removed part of the previous cooling effect of the aerosols and allowed the underlying global warming trend to emerge
• In contrast, a simultaneously abrupt and total end to anthropogenic CO2 emissions would lead to a much slower reduction in atmospheric CO2 levels.
  • The ocean would take up half of the pulse of excess CO2 that had been produced by previous human activities within 50 years or less, but 10–20% of the total pulse of emissions would remain in the atmosphere for many millennia.
  • As a result, part of the anthropogenic warming would persist as long as this extended tail end of the excess CO2 pulse lingered in the atmosphere.
  • The net effect of reducing aerosol and CO2 emissions would be additional warming.
• Atmospheric chemist Paul Crutzen proposed that the warming effect of greenhouse gases could be countered by sending sulfate aerosols high into the stratosphere.
• Because gravity pulls stratospheric aerosols down into the troposphere within a few years, sustaining a high enough concentration at those altitudes to offset global warming would require a constant supply of aerosols, by one estimate half a million tons per year.
• Another proposal is to send trillions of tiny mirrors into orbit to reflect a few percent of the incoming solar radiation back into space.
• Yet another idea is to reduce the amount of black carbon soot that settles on snow and sea ice at high Arctic latitudes. These particles accelerate melting of ice and snow because of their dark, heat-absorbing properties. Because most black carbon in the Arctic comes from annual burning of crops and pasture grasses at high latitudes, the darkening effect of these aerosols could be reduced by delaying burning until after most of the late-spring snow melted.
• Because all of the proposed climate-engineering mechanisms cost money, each will face hard questions about cost-effectiveness.

Epilogue

• Estimated future changes in climate will likely reach a size comparable to the largest natural changes of the past.
• The projected 5C average global warming in a 3 x CO2 world would match the 5C cooling at the most recent glacial maximum 20,000 years ago. We are now, 0.8C of the way toward that ,5C warmer future.
  • Unless technology or extreme conservation efforts intervene, we are likely to reach that much warmer future at rates that are unprecedented in Earth’s 4.5 billion year climatic history.
• In the distant future, most of the pulse of excess CO2 will disappear into the ocean, and climate will cool back toward its preindustrial level, but not all the way back. The rate of decline will depend mostly on how much of the carbon reservoir buried in the ground we extract between now and then.
• Part of the excess CO2 pulse (about 15%) will linger in the atmosphere for tens of thousands of years and keep climate too warm for new ice sheets to accumulate on North America or Eurasia.
• From the perspective of Earth’s past climatic history, it is astounding to realize that we have now put an end to northern hemisphere glacial cycles that had been underway for almost 3 million years.
• What other major transformations will we cause in the future?