I am reading Chapter 19 this week. All notes were things I highlighted in my book.

Chapter 19 Causes of Warming over the Last 125 years

Natural Causes of Recent Warming

• A key question in the 0.88C global warming since the late 1800s is the role of natural changes in climate. Here, we examine possible sources of natural variations in climate during the last 125 years, proceeding from the longer-term to the shorter-term factors.

19-1 Tectonic, Orbital, and Millennial Factors

• changes in climate over tectonic time scales are irrelevant to the changes of the last 125 years.
  • During the transition from greenhouse (ice-free) conditions to the current icehouse state, Earth’s climate cooled by at most 5C to 10C over 50 to 100 million years. The average rate of cooling (~0.00001C per century) falls far short of the rates needed to alter climate measurably in 125 years.
• Changes in Earth’s tilt and precession over orbital time scales have altered the amount of insolation received at different latitudes and in different seasons, but orbital forcing is also not a viable explanation of the recent warming.
• Global average temperature during the last 6,000 years has cooled by less than 1C, at an average rate of 0.00016C per century or less, much too slow to account for the 0.8C warming during the last century, as well as opposite in direction
• Millennial-scale oscillations were large when northern ice sheets existed, but they weakened as the ice melted.
  • During the last 8,000 years of the current interglaciation, climatic oscillations have been small and highly irregular in character from region to region. In contrast, the warming of the last 125 years has been globalin extent, with ever-fewer regions trending counter to the warming pattern.
  • This global response does not match the pattern of the glacial millennial oscillations, with Antarctic responses opposite in sense to those in the North Atlantic region. Millennial-scale oscillations do not appear to be a factor in the recent warming.

19-2 Century and Decadal-Scale Factors: Solar Forcing

• Satellite measurements of the amount of radiation arriving from the Sun began in 1978 and now extend over three 11-year cycles. During those cycles, solar radiation has varied by a little over 1 W/ m2 , equivalent to about 0.1% of the global average value. Prior to 2011, satellites had indicated a global average value of 1,365–1,366 W/m2 , but improvements in technology have now reduced that value to 1,361–1,362 W/m2 . The amplitude of the 11-year cycles remains very nearly the same.
• Climate models indicate that a change of 0.1% in the strength of radiation arriving from the Sun could alter global mean temperature by as much as 0.2C if it persisted for many decades. Half of an 11-year cycle, however, does not give the climate system enough time to register its equilibrium response.
  • As a result, Earth’s mean temperature in the models warms and cools by less than 0.1C in response to the 11-year solar variations. Temperature changes this small are difficult to distinguish from the natural variability in Earth’s climate system, and very few observational records show convincing evidence of an 11-year temperature signal.
• As noted in Chapter 17, until a few years ago scientists estimated that the Sun may have been 0.25–0.4% weaker than it is now during several long sunspot minima that occurred prior to the industrial era, such as the Maunder minimum (1645–1715). Such a change would be two to three times the range measured by satellites since 1979. If changes this large persisted for decades, the climate system would have had time to come much closer to its full equilibrium response. Model simulations at that time indicate that temperatures could have risen by as much as 0.3C to 0.4C above those during the 1800s. If so, almost half of the 0.8C warming during the 1900s might have been accounted for by solar forcing.
• This hypothesized Sun–climate link has not held up well under closer scrutiny because observations of Sun-like stars have not detected variations over these longer multidecadal intervals.
  • Current estimates suggest that changes in solar irradiance have accounted for less than 0.1C of the 0.8C warming since the late 1800s. During the 35 years of direct satellite observations, when temperatures have climbed especially rapidly, the solar contribution has been negligible.
  • A proposed Sun–climate link operating through the stratosphere remains a possibility, but it remains largely speculative.

19-3 Annual-Scale Forcing: El Nino and Volcanic Eruptions

• Two factors have had measurable effects on global climate over brief intervals lasting about a year: major El Niño events and large volcanic explosions.
  • During the last century, both factors have altered global temperature by less than 1C, and their effects have disappeared into the background noise of the climate system within a year or two.
• Large El Niño episodes can warm the eastern tropical Pacific Sea surface by 2C to 5C and raise average global temperature by 0.1C to 0.2C. The major El Niños that occurred in 1983 and 1998 produced one year warm spikes in temperature, but their effects disappeared by the following year.
• Several large volcanic eruptions occurred during the era of instrumental temperature records, including Katmai (1912), Agung (1963), and El Chichón (1981). Although these eruptions probably cooled global climate by 0.1C for a year or two
  • the amount of cooling is difficult to determine because of uncertainties about the sulfur content of each eruption and the height in the atmosphere to which the SO2 was injected.
• The large Mount Pinatubo eruption in the Philippines in 1991 was the first volcanic explosion measured in sufficient detail to assess its effect on global climate.
  • Global climate cooled by 0.6C during the summer after the eruption and by an average of ,0.3C for the first full year after the eruption.
  • By the second year, the cooling effect of Pinatubo disappeared into the background noise of natural year-to-year temperature variability.
  • If the brief effects of the warm El Niño years and the cool volcanic years are removed from the temperature records, the steady background warming trend of the last decades becomes even more obvious.

Anthropogenic Causes of the recent Warming

• With natural causes all but ruled out, the bulk of the recent warming must be anthropogenic in origin. Here we explore several kinds of emissions from human activities that have altered climate: the greenhouse gases carbon dioxide and methane, chlorine bearing chemicals of various kinds, and sulfate and carbon aerosols.

19-4 Carbon Dioxide (CO2)

• Measurements by the geochemist Charles Keeling begun in 1958 show an accelerating rise in the CO2 concentration during the last two centuries. By 2010 the CO2 concentrations had risen to 390 ppm, well above the 180–300 ppm range of natural variations.
• The additional carbon emitted from human activities has come mainly from two sources.
• Throughout the late 1700s and most of the 1800s, the main source of carbon was clearing of forests to meet human needs: farmland for agriculture, wood for home heating, and charcoal to fuel the furnaces of the early part of the Industrial Revolution.
• After 1900, most of the extra carbon added to the atmosphere came from extraction of fossil fuels buried beneath Earth’s surface.
• Today, industrial carbon emissions account for most of the fossil-fuel total, while cutting and burning of tropical rain forests are the largest land clearance contribution. 
• In recent decades, 55% of the excess carbon produced each year has ended up in the atmosphere and another 25–30% has been added to the ocean.
• The excess CO2 from human activities has already been well mixed into the upper tens of meters of the ocean, which quickly exchange molecules of gas with the atmosphere.
• Huge numbers of measurements made over vast regions of the ocean during all seasons are required to quantify the slow penetration of CO2 into and beneath the ocean surface.
  • Despite these problems, growing numbers of measurements confirm that the ocean takes up about 25–30% of the total human carbon input.
• If 55% of the excess carbon ends up in the atmosphere and 25–30% in the oceans, where does the other 15–20% go?
  • The only major reservoir left is the biosphere, both live vegetation and dead organic litter in soils and coastal estuaries
• Across nearly all of eastern North America, forests had been almost completely cut by the late 1800s for farming and for fuel.
  • Rock walls that once marked the boundaries of open fields now run through growing forests. Near eastern cities, rural areas that had been cleared of trees gradually turned into tree-shaded suburbs. This widespread regrowth of trees has extracted CO2 from the atmosphere.
• A  way to remove CO2 from the atmosphere is through CO2 fertilization.
  • Vegetation uses CO2 during photosynthesis to create the cellulose that forms leaves, blades of grass, tree trunks, and roots.
  • Greenhouse experiments show that most plants obtain carbon more easily from a CO2-rich atmosphere and grow faster as a result.
•  Scattered evidence suggests that the 35% rise in atmospheric CO2 in the last 200 years has increased this fertilization effect and taken more carbon from the atmosphere. The vegetation grows faster; it becomes more varied in composition and grows more densely; the amount of woody material in tree branches, trunks, and roots increases; and trees and shrubs shed more fresh carbon litter into soils and coastal estuaries.

19-5 Methane (CH4)

• The concentration of the greenhouse gas methane in the atmosphere has also increased as a result of human activity. The influence of humans is again evident from trends measured both in ice core air bubbles and from instrumental observations.
  • Since the 1800s, the methane concentration has risen to over 1,750 ppb, above the natural range of 350–700 ppb during the previous 800,000 years (ice cores)
• Methane comes from sources that are rich in organic carbon but lacking in oxygen.
  • In the absence of oxygen, bacteria break down the vegetation and extract the carbon, which combines with hydrogen to form methane gas.
• Increased emissions of CH4 during the last 200 years have resulted from the explosion in human population.
  • Ever-increasing areas of tropical land have been put into rice paddy cultivation in Southeast Asia, and these artificial wetlands produce methane.
  • The growing numbers of cattle and other livestock have increased the amount of methane gas sent to the atmosphere.
• Today, the amount of CH4 produced by these and other human activities is more than twice as large as that from natural sources.
• Methane also acts as a greenhouse gas by trapping outgoing radiation from Earth’s surface.
• Although its concentration is much lower than that of CO2, it is far more effective on a molecule-by-molecule basis in trapping radiation than CO2. The enormous increase in methane concentration during the last two centuries has also caused the planet to warm.

19-6 Increases in Chlorofluorocarbons

• CFCs, or chlorofluorocarbons contain elements chlorine (Cl), fluorine (Fl), and bromine (Br). CFCs have for decades been produced for use as refrigerator and air conditioner coolants, chemical solvents, fire retardants, and foam insulation in buildings. Released at ground level, they are slowly mixed upward through Earth’s atmosphere. Because CFCs stay in the atmosphere for an average of 100 years, they eventually reached the stratosphere, where their concentrations increased during the late 1900s.
• Ozone (O3) occurs naturally in the stratosphere, with the largest concentrations at altitudes between 15 and 30 km.
• Incoming ultraviolet (UV) radiation from the Sun liberates individual O atoms from oxygen (O2) and produces ozone: Radiation (UV) + O2 -> O + O O + O2 -> O3 (ozone)
• Ozone is naturally converted back to oxygen (O2) in the atmosphere by a similar process, but in this case the radiation source can be light in either ultraviolet or visible wavelengths: O3 + Radiation (UV or Visible) -> O2 + O O + O3 -> 2O2
• With visible radiation far more abundant than ultraviolet radiation, ozone is normally destroyed much faster than it is produced. As a result, ozone is a short-lived gas.
  
  • Chlorine reacts with ozone and destroys it, forming chlorine monoxide (ClO): Cl + O3 -> CLO + O2. Chlorine then reacts with free oxygen molecules and is liberated from ClO: CLO + O -> CL + O2
• These liberated chlorine atoms then begin a new cycle of ozone destruction. This cycle is important to humans because ozone in the stratosphere forms a natural protective barrier that shields life-forms from levels of ultraviolet radiation that would otherwise produce cell mutations including skin cancers.
• In the last century, humans have greatly accelerated the natural destruction of ozone by adding extra chlorine to the stratosphere. Measurements from 1979 to 1990 showed that the amount of ozone in a column of air over Antarctica had decreased considerably in the region where stratospheric chlorine is unusually abundant.
• This clear connection between CFC emissions and ozone depletion caused so much alarm that the world’s nations signed a treaty in Montreal in 1987 to reduce and ultimately eliminate the use of CFCs.
•  Stratospheric ozone levels have now stopped falling, but have not yet begun a major recovery toward natural levels. Rebounds in ozone concentration are expected to begin within a decade or two.
• Ozone also occurs naturally in much greater abundance in the lower troposphere. It originates from both natural and anthropogenic processes, including biomass burning and oil production in refineries.
  • At these lower levels in the atmosphere, ozone generally plays a positive environmental role by cleansing carbon monoxide and sulfur dioxide from the air
•  At high concentrations, however, ozone is toxic to plants and an irritant to human eyes and lungs. In the lowermost atmosphere, ozone trends have moved in the opposite direction during the industrial era. Human activities have caused large ozone increases that have produced periodic smog alerts in many large cities. Slow-moving air masses settle over urban areas in summer and allow concentrations of ozone and other pollutants to build to levels that are dangerous to humans with respiratory problems.

19-7 Sulfate Aerosols

• Industrial era smokestacks emit the gas sulfur dioxide (SO2) as a by-product of smelting operations in furnaces and from burning coal. SO2 reacts with water vapor and is transformed into sulfate particles, called sulfate aerosols.
  • Because these aerosols stay within the lower several kilometers of the atmosphere, their primary impact on climate is regional in scale
• Until the 1950s, smokestacks in Europe and North America were small and most SO2 emissions stayed close to ground level, producing thick industrial hazes and sulfur-rich acidic air that etched building facades and cemetery monuments made of limestone and marble.
  • In the 1970s, taller smokestacks were built to disperse SO2 emissions higher in the atmosphere. This effort dramatically improved air quality in many cities but created a different problem in more distant areas.
• The sulfate particles that are now being sent higher in the atmosphere are carried by fast-moving winds across broad areas.
  • Although sulfates stay in the atmosphere for only a few days before rain removes them, they can be carried 500 or more kilometers downwind from source regions.
• Greenland ice contains a record of industrial SO2 emissions from regions upwind. Throughout the 1800s, industrial SO2 emissions had been smaller than those from natural sources, but by the middle 1900s industrial emissions had become the dominant SO2 source. Concentrations then began to drop sharply after 1980, when the United States acted to limit its SO2 emissions.
• Climate scientists have inferred that the large plumes of sulfate aerosols cause regional cooling downwind from smokestack sources.
  • Like the sulfate aerosols created by volcanic explosions, industrial sulfate particles reflect and scatter some of the incoming solar radiation back to space and keep it from reaching Earth’s surface. The reduction in radiation cools climate regionally, and these effects show up in tabulations of global mean temperature change.
• A second potential climatic effect of sulfate aerosols is less well understood. Tiny aerosol particles form centers (nuclei) around which water vapor condenses, forming droplets and then clouds. Clouds can have two opposing effects on climate:
  • the surfaces of thick clouds can reflect more incoming solar radiation and cool climate, but thinner higher clouds can absorb more outgoing radiation from Earth’s surface and increase the greenhouse effect.
  • Because sulfate aerosols mainly affect low-level clouds, their indirect effects appears to cool climate, but the amount of cooling is highly uncertain.

19-8 Brown Clouds, Black Carbon, and Global Dimming/Brightening

• In addition to sulfate aerosols, human activities send a wide range of other pollution into the atmosphere. Anyone traveling on airplanes can see low-lying hazes hanging over many regions, including large cities and rural regions where burning is frequent.
  • These haze layers contain a wide range of physical and chemical constituents generated by human activities such as biomass and coal burning, diesel exhaust, forest fires and cook stoves. The resulting climatic effects vary widely
• Dark carbon-rich aerosols absorb the Sun’s radiation and warm the air layer in which they lie. Scientists initially suspected that these hazes heated the lower atmosphere and added to the net amplitude of global warming on a regional basis.
  • But investigations during the late 1990s and early 2000s led to a surprisingly different interpretation of hazes in Southeast Asia called brown clouds.
  • Most of the carbon-rich aerosols in these hazes come from small cook stoves in which people burn organic matter for fuel, including cow dung. The aerosols in these hazes do absorb radiation, but their effects are concentrated in a layer of air two to three kilometers above the surface.
• Although this layer warms, the haze blocks a portion of the incoming solar radiation and prevents it from reaching Earth’s surface, which cools. In regions of severe brown-cloud hazes, the reduction in solar radiation during peak seasons is almost an order of magnitude larger than the global average increase from the greenhouse-gas effect. The effects of the brown clouds have also been found to extend thousands of kilometers downwind from source regions.
• In contrast, particles of black carbon produced by various kinds of burning can have a warming effect on climate in far-northern regions.
  • When dark particles fall out of the atmosphere onto bright surfaces of snow and sea ice, they absorb solar radiation that would otherwise have been reflected. This absorbed heat melts snow and sea ice, in some cases exposing the darker land and ocean surfaces below, which further warm regional climate by albedo feedback.
• Some estimates place the global warming role of black carbon at levels near or above that from methane, but it remains difficult to evaluate this warming role against the cooling effect of brown clouds. The role of black carbon in “seeding” aerosols that form clouds is also unclear.
• During the 1950s to 1980s, measurements taken at Earth’s surface in many mid-latitude regions of the Northern Hemisphere detected a phenomenon called global dimming, in which the amount of solar radiation reaching the ground slowly decreased.
  • Toward the end of that period, satellite measurements of the brightness of Earth’s surface from space confirmed a decrease of several percent. (result of sulfate aerosols, brown clouds, contrails emitted by jets, and other emissions)
  • The cooling caused by reduced solar radiation is thought to have been large enough during the 1940s to 1970s to cancel the greenhouse-gas warming trend during that interval.
• Since the 1980s, aerosol emissions in several industrial regions (excluding India and China) have been reduced, reversing the long-term dimming trend and allow the surface to brighten. This trend toward increased solar radiation is likely responsible for part of the rapid warming during that interval. But the dimming trend continues over Southeast Asia and other regions where emissions from urbanization and other sources like biomass burning are still growing.

19-9 Land Clearance

• Human activities, primarily the cutting of forests for agriculture, have altered much of the land surface of the planet.
  • At high and middle latitudes, forest clearance causes a net increase in albedo, as darker forests are replaced by brighter pastures and croplands. With more solar radiation reflected back into space, the surface cools.
  • At tropical and subtropical latitudes, forest clearance also reduces the amount of evapotranspiration. With reduced moisture availability, land surfaces dry out and bake in the intense summer Sun, warming regional climate.
• Trends of land clearance during the last 150 years have been complex. Most active deforestation in recent decades has occurred in tropical South America and tropical islands of Southeast Asia, but most northern hemisphere regions that were deforested until the 1800s or early 1900s have been reforesting for many decades.
• On a global average basis, the net effect of land clearance has been a small cooling of the planet.

Earth’s Sensitivity Greenhouse Gases

• Our understanding of past, present, and future climate hinges in part on Earth’s sensitivity to greenhouse gases. Two independent sources of evidence put constraints on this sensitivity:
  • numerical models of the climate system (mainly general circulation models)
  • climate reconstructions of intervals from Earth’s history when greenhouse-gas concentrations differed from those today.

19-10 Sensitivity in Climate Models

• For convenience, Earth’s sensitivity to changes in greenhouse gases is often quantified as the global average change in surface temperature caused by a doubling of CO2 concentrations beyond the modern (preindustrial) level of 280 ppm.
• The CO2 concentration has increased by 40% since the start of the industrial era. Climate scientists also include the combined heat-trapping effects of the other greenhouse gases by converting changes in these gases to equivalent changes in CO2.
• Calculating the potential effect of the greenhouse gas increases on Earth’s climate involves two steps.
• Step 1 is to determine the radiative forcing provided by the gases, using the same W/m2 units with which incoming solar radiation and Earth’s back radiation are measured.
  • This radiative forcing excludes the complicating effect of the many natural feedbacks operating in the climate system. (Clear sky)
  • A range of observational and modeling evidence indicates that doubling CO2 levels would increase global temperature by 1.2C through this radiative effect alone.
• Step 2 is to assess the effects of natural feedbacks within the climate system that can either amplify or oppose the radiative warming. Positive feedbacks and negative feedbacks.
• Together, these two factors determine Earth’s average temperature sensitivity to changes in greenhouse gases. General circulation model experiments over many decades have produced a wide range of estimates of the temperature sensitivity to the 2 x CO2 level. These estimates have generally varied between 2C and 5C.
• Uncertainties about the size of feedbacks in the climate system are the reason for this wide range of estimates. The most prominent feedbacks come from changes in water vapor, in the albedo of snow and ice, and in clouds and cloud aerosols.
• Water vapor, the major greenhouse gas in Earth’s atmosphere today, provides positive feedback to the warming initiated by increases in greenhouse gases.
  • Current estimates are that the initial 1.2C radiative global warming caused by doubling CO2 levels should cause an additional warming of between 1.1C and 1.5C through water vapor feedback.
• Another positive feedback results from reflection of incoming solar radiation by bright surfaces covered by sea ice and snow.
  • If a warming initiated by greenhouse gases causes a retreat of snow and ice toward the poles, the reduced extent of these high-albedo surfaces will increase the absorption of solar radiation at high latitudes and warm them further. The resulting positive feedback should increase the initial 2 x CO2 global warming by about 0.6C.
• Feedback from clouds is the major uncertainty in climate models. Different types of clouds vary in the amount of solar radiation they reflect compared to the amount of back radiation from Earth’s surface they absorb and retain in the climate system.
  • High, wispy clouds tend to warm Earth’s climate, because they are composed of ice crystals that are better at absorbing outgoing radiation than at reflecting incoming radiation.
  • Thicker, lower clouds cool climate because they are better at reflecting incoming solar radiation than at trapping outgoing radiation. The cooling effect of lower clouds is obvious in our daily lives; the warming effect of high clouds is not.
• The problem facing scientists is assessing all the changes in the many types of clouds as Earth’s climate warms.
• As a result of all these uncertainties, the net feedback from clouds remains highly uncertain. If the 2 x CO2 climate sensitivity is 3C, then the net feedback from clouds should lie close to zero. But considering the full range of possible climate sensitivities, the cloud feedback could either be strongly negative(2C) or strongly positive (5C).

19-11 Sensitivity to Greenhouse Gases: Earth’s Climate History

• Earth’s climatic history provides additional constraints on Earth’s sensitivity to CO2 changes. The preindustrial temperature and CO2 concentration represent just one point along a continuous curve of possible climatic states.
  • For a given amount of change in CO2, larger temperature responses occur toward the low-CO2 end of the range than at the high-CO2 end.
• A major reason for this varying sensitivity is the greater extent of snow and ice cover when CO2 concentrations are lower. The increased areas of snow and sea ice in these colder climates provide a larger albedo-temperature feedback to the initial CO2 changes. In contrast, at the high-CO2 end of the range, snow and ice are reduced in extent, and little albedo feedback occurs.
• A second reason for the varying sensitivity is that the trapping of Earth’s back radiation becomes less efficient at higher CO2 concentrations as the atmosphere gradually becomes saturated with CO2.
• The model used to generate had a 3C increase in global temperature for a CO2 doubling from 280 to 560 ppm.
  • The overall range of uncertainty shown by other GCM models can be tested by examining intervals from Earth’s climate history when past CO2 levels and global mean temperature are either well known or can be reasonably well constrained.
  • The temperature changes at these times should represent the response to changes in CO2 and other greenhouse gases.

The Last Glacial Maximum

• The most informative interval from the past is the last glacial maximum, 21,000 years ago. At that time atmospheric CO2 values were near 190 ppm, about 30% lower than the preindustrial level of 280 ppm, and methane values were 350 ppb, about 50% lower than their preindustrial value. The methane reduction translates into a 15% loss in CO2, bringing the net drop in equivalent CO2 concentration to 45%.
• Estimates of the amount of glacial cooling in the tropics range from as small as -1.5C (CLIMAP) to as large as -4C or -5C. This range of estimates indicates that Earth’s sensitivity to CO2 largely falls within the range indicated by climate models, but toward its higher end.
• Incorporating the cooling effects of heavy glacial dust concentrations in the atmosphere and higher albedo due to reduced vegetation would bring the range of estimated sensitivities for the last glacial maximum within the -2C to -5C range.

The Cretaceous (100 million Years Ago)

• The Cretaceous world of 100 million years ago can also be compared to the modern world. Estimates of greater warmth during this greenhouse interval range from +5C to +11C, with recent estimates favoring the middle end of this range.
• Unfortunately, CO2 values for the Cretaceous are not tightly constrained. Several techniques based on analysis of carbon isotopes in the remains of fossil plants and soils suggest values considerably higher than those today, but the estimates range from four to twelve times the preindustrial CO2 level.
• With Cretaceous temperatures warmer by substantially more than 5C, Earth’s sensitivity appears to fall toward the high end of the wide range of uncertainty.
• In Summary, analyses of past intervals generally support the range of CO2 sensitivity estimated from climate models. For the best-constrained case, the most recent glacial maximum, the inferred sensitivity largely lies within the range of model estimates.

Why Has the Warming Since 1850 Been So Small?

• The evidence summarized to this point has shown that natural factors have played a small role in the global warming trend since 1880 and that greenhouse gases have been the dominant driving factor.
• Yet some climate skeptics continue to resist this conclusion based on the small size of the observed global warming (0.8C) compared to the large (40%) rise in CO2, and the even larger (65%) rise in equivalent CO2.
  • They argue that an increase in greenhouse gases of this magnitude should have warmed Earth’s surface by ,1.6C to 1.7C, far more than the 0.8C increase actually observed.
• Based on this analysis, they claim that Earth’s sensitivity to CO2 and other greenhouse gases is well below the lower end of the range indicated by climate models. But mainstream climate scientists have shown that this analysis is invalid because it ignores two other factors that have also affected global temperature since 1880.

19-12 Delayed Warming: Ocean Thermal Inertia

• One important factor is the effect of thermal inertia in delaying the full response of the climate system to greenhouse-gas emissions in recent decades.
• When external factors cause climate change, some parts of the climate system react more slowly than others because of their greater thermal inertia.
• The most important source of thermal inertia for the climate system over intervals of a century or so is the ocean, which covers 70% of Earth’s surface and stores enormous amounts of heat.
  • The upper layer of the ocean is stirred by lower-atmospheric winds and acts as a relatively fast responding part of the climate system within a decade or two.
  • The ocean layers lying several hundred meters below the wind-mixed layer and less affected by changes at the surface still play a role in the ocean’s response, but over longer time intervals (50–75 years).
    • As a result of this slower response, the amount of increase in global surface temperature measured at any particular time within the last 125 years represents only a part of the warming that is eventually going to occur when those deeper layers respond, even with no further increase in gas concentrations
•  The implication of this delayed warming is that Earth’s 2 x CO2 sensitivity must be considerably larger than the value that would be derived by comparing the CO2 increase between the 1800s and the early 2000s against the amount of warming that took place during that interval.
• Because the rise in greenhouse-gas concentrations during the last half-century has been especially rapid, some of the warming they will eventually cause is still “in the pipeline” (not yet realized).

19-13 Cooling from Anthropogenic Aerosols

• In the late 1970s, the climate scientist Murray Mitchell proposed that the size of the greenhouse-gas warming was being significantly reduced by an offsetting cooling effect caused by smokestack emissions of SO2, leading to production of sulfate particles in the atmosphere.
• At that time, SO2 emissions in most areas were rising with an exponential trend similar to the CO2 emissions because both were by-products of the era of heavy industrialization. Today, three major sulfate plumes lie over and downwind of the eastern United States, Eastern Europe, and China.
• Current estimates are that the direct effect of sulfate aerosols from smokestacks and carbon-rich aerosols from biomass burning and cook stoves have reduced incoming solar radiation enough to offset about 25% of the radiative forcing caused by greenhouse-gases.
• Another potentially larger negative feedbacks arise from the indirect effect of aerosols in seeding cloud nuclei. By increasing low-level cloud cover, this feedback could promote surface cooling, but the amplitude is highly uncertain. Ignoring these offsetting cooling effects from various anthropogenic activities makes the sensitivity of the climate system appear smaller than it actually is.