Considering the extremes of Greenhouse Effects on Mars and Venus, we can better appreciate the precise balance which allows our own atmosphere to provide temperatures hospitable to liquid water and life. Inevitably, we must also ask this chapter’s repeating query: how have human activities affected this equilibrium? This leads us back to the 2007 Nobel Peace Prize, and an evolving consensus that our species is responsible for significant global warming.
Global Warming
Global warming refers to the recent increase in the Earth’s average near-surface and ocean temperatures (Figure below). During the past 100 years, surface air temperatures have risen 0.74 ± 0.18 °C (1.33 ± 0.32 °F). Multiple sources agree that the two warmest years since the introduction of reliable instrumentation in the 1800s were 1998 and 2005.
Figure 18.52
Global warming refers to the increase in Earths average near-surface temperatures over the past 100 years. Anomalies measure deviation from 1961-1990 averages.
This recent increase contrasts with relatively stable temperatures shown by scientific data for the previous two millennia. Multiple sets of temperature data inferred from tree rings, coral growth, and ice core samples are compiled in Figure below. Warmly debated exceptions to the stability include a warm period during the Middle Ages and a “Little Ice Age,” attributed to decreased solar activity and increased volcanism.
Figure 18.53
Global temperatures compiled from tree ring, coral growth, ice core analysis, and historical records, show relative stability over the last 2,000 years before about 1850, interrupted by a debatable Medieval warming and a more recent cooling termed the Little Ice Age. Colored lines indicate different published data sources. For more detail on the increase since 1850, refer to Figure 3.
According to paleoclimatologists, on a scale of millions of years Earth’s temperatures have varied almost regularly (over time intervals of roughly 140 million years) from those which support global tropics to continental glaciations (Figure below). Scientists estimate the global average temperature difference between an entirely glaciated Earth and an ice-free Earth to be 10oC.
Figure 18.54
Paleoclimatological measures of global temperatures show dramatic fluctuations in temperature. Graphs should be read from right (past) to left (present). Ice core data for temperature is recorded in oxygen isotope units rather than C.
The causes of Ice Ages are not completely understood, but greenhouse gases, especially CO2 levels, often correlate with temperature changes (Figure below). Rapid buildup of greenhouse gases in the Jurassic Period 180 million years ago correlates with a rise in temperature of 5oC (9oF). Similar changes have been hypothesized as causes for the dramatic Permian Extinction 250 million years ago and the Paleocene-Eocene Thermal Maximum (one of the most rapid and extreme global warming events recorded in geologic history) 55 million years ago. Paleoclimatologist William Ruddiman proposes that human activities began to affect global CO2 levels as long ago as 8,000 years, when agriculture and deforestation began. Ruddiman argues that without this early contribution to greenhouse gases, cycles indicate the Earth would already have entered another Ice Age.
Figure 18.55
Over the past 450,000 years, temperature changes (blue) correlate closely with changes in atmospheric CO (green) and dust levels (red).
Others dispute Ruddiman’s “overdue-glaciation” theory, but most scientists today agree that recent global warming since 1850 is caused by an unprecedented rise in atmospheric CO2 (Figure below) which resulted from human activities – primarily burning of fossil fuels, but also continuing deforestation and changes in land use. Fossil fuels burn organic compounds in the same way your cells burn glucose to make ATP: a product of both reactions is CO2. Deforestation and other land use changes contributes to the CO2 levels from the opposite direction – a decrease in photosynthesis, which would have removed CO2 from the atmosphere. Slash-and-burn destruction of tropical forests combines the worst of both worlds; burning adds CO2 to the atmosphere, and the loss of layers of vegetation decreases CO2 use.
Two additional greenhouse gases having anthropogenic (human activity) sources are methane (CH4) and nitrous oxide (NO). Agriculture adds both of these to the atmosphere; cattle production is responsible for much of the methane, a powerful greenhouse gas. Land use changes, waste processing, and fossil fuel production, which we’ve already implicated in CO2 increases, are other anthropogenic (human-caused) sources. A last but important contributing factor is secondary to these primary causes; triggers of “runaway greenhouse effects” will be discussed below.
Figure 18.56
Since the Industrial Revolution began, the burning of fossil fuels has dramatically increased atmospheric concentrations of CO to levels unprecedented in the last 400 thousand years. The graph on the right integrates recent measurements with paleoclimatologic data.
Although the causal connections between fossil fuel combustion, deforestation, greenhouse gases, the greenhouse effect, and global warming have been strongly debated in the past, the majority of the world’s scientific organizations now support these relationships, and many use the term “consensus.” (See “Scientific Opinion about Climate Change” in Further Reading.) The awarding of the Nobel Peace Prize to the organization which focuses most directly on climate change, the IPCC, highlights this consensus. Alternative hypotheses include variation in solar activity; several references are included in Further Reading. The IPCC projects future temperature increases ranging from 1.1 °C to 6.4 °C (2.0 °F to 11.5 °F) between 1990 and 2100. Predictions from multiple models which incorporate connections between greenhouse emissions and global warming are summarized in Figure below; all show significant rises in temperature by 2100.
Figure 18.57
Various models of climate change which include business-as-usual increases in greenhouse gas emissions predict continuing increases in global temperature; this graph compares the projected increases to temperatures during the year 2000.
Once again, then, we “have met the enemy” and “he is us.” What have we done? What are the environmental and socioeconomic consequences of this human disruption in atmospheric equilibrium?
A partial list of effects of climate change includes:
Direct Physical Effects
Melting of glaciers and a consequent rise in sea level, already documented (Figure below)
Sea level rise of 18-59 cm predicted by 2100
River flooding followed by drought
Coastal flooding and shoreline erosion
Figure 18.58
Glacial melting (left) and a rise in sea level (right) are two consequences of global warming. The left image shows the Larsen Ice Shelf B, which broke up during February of 2002 after bordering Antarctica for as long as 12,000 years. Excluding polar ice caps, 50% of glacial areas have disappeared since the turn of the century. Although sea levels have risen since the end of the last Ice Age, rates increased by a factor of 10 beginning about 1900.
Melting permafrost, leading to release of bog methane (CH4) increasing warming via positive feedback*
Changing patterns of precipitation
Regional drought
Regional flooding
Ocean warming, leading to increased evaporation
Increasing rainfall
Increasing erosion, deforestation, and desertification
Release of sedimentary deposits of methane (CH4) hydrates – positive feedback*
Ocean acidification: 0.1 pH unit drop already documented; 0.5 more predicted by 2100
Loss of corals
Loss of plankton and fish
Temperature extremes
Increasing severity of storms such as tropical cyclones, already documented (Figure below)
Further reductions in the Ozone Layer (due to cooling of the stratosphere)
Figure 18.59
The proportion of hurricanes reaching category 4 or 5 increased from 20% in the 1970s to 35% in the 1990s. The EPA and the World Meteorolo
gical Organization connect this increase to global warming, and NOAA scientists predict a continuing increase in frequency of category 5 storms as greenhouse gases rise.
Ecosystem Effects
Contributions to the Sixth Extinction reaching as much as 35% of existing plant and animal species
Decline in cold-adapted species such as polar bears and trout
Increase in forest pests and fires
Change in seasonal species, already documented
Potential increase in photosynthesis, and consequent changes in plant species
Loss of carbon to the atmosphere due to
Increasing fires, which together with deforestation lead to positive feedback
Increasing decomposition of organic matter in soils and litter
Socioeconomic Threats Result From Some of the Above Changes
These include:
Crop losses due to climate and pest changes and desertification
Increasing ranges for disease vectors (e.g., mosquitoes – malaria and dengue fever)
Losses of buildings and development in coastal areas due to flooding
Interactions between drought, desertification, and overpopulation leading to increasing conflicts (Figure below)
Figure 18.60
A camp in Sudan houses refugees from the far western province of Darfur, who fled from genocide intensified by severe drought. The Darfur conflict echoes predictions that global warming may increase drought and desertification in overpopulated regions and result in more such tragedies.
Costs to the insurance industry as weather-related disasters increase
Increased costs of maintaining transportation infrastructure
Interference with economic development in poorer nations
Water scarcity, including pollution of groundwater
Heat-related health problems
Threats to Political Stability
Migrations due to poverty, starvation, and coastal flooding
Competition for resources
Note that at least three(*) of the direct physical effects – melting permafrost, ocean warming, and forest fires/deforestation - can potentially accelerate global warming, because temperature increases result in release of more greenhouse gases, which increase temperatures, which result in more greenhouse gases – a positive feedback system aptly termed a “runaway greenhouse effect.” Here’s how it could work: rising temperatures are warming the oceans and thawing permafrost. Both oceans and permafrost currently trap huge quantities of methane – beneath sediments and surface – which would undergo massive releases if temperatures reach a critical point. Recall that methane is one of the most powerful greenhouse gases, so the next step would be further increase in temperatures. Warmer oceans and more thawed permafrost would release more quantities of methane – and so on. These compounding effects are perhaps the most convincing arguments to take action to reduce greenhouse gas emission and global warming.
What measures have been considered?
Preventing Climate Change
Basically, greenhouse gases are products of fossil fuel combustion; according to the EPA, more than 90% of U.S. greenhouse gas emissions come from burning oil, coal, and natural gas. Therefore, energy use is the primary target for attempts to reduce future global warming. In Figure below you can see the sources of emission for three major greenhouse gases in 2000, when CO2 was 72% of the total, CH4 18%, and NO 9%. Chlorofluorocarbons (CFCs, HCFCs, and HFCs) are also greenhouse gases; refer to the lesson on The Atmosphere for more information about them.
Figure 18.61
Global greenhouse emissions during 2002 show sources for each of the three major greenhouse gases. Knowing the causes makes finding solutions clear, but not necessarily easy!
Knowing the causes of climate change allows us to develop potential solutions. Direct causes include combustion of fossil fuels, deforestation and other land use changes, cattle production, agriculture, and use of chlorofluorocarbons. Runaway effects can result from temperature-dependent release of methane from permafrost and ocean sediments, and forest fires or intentional burning. Unfortunately, the best way to avoid runaway effects is to prevent temperature increases. Prevention, then, should address as many of these causes as possible. A partial list of solutions being considered and adopted follows.
Reduce energy use.
Switch to cleaner “alternative” energy sources, such as hydrogen, solar, wind, geothermal, waste methane, and/or biomass.
Increase fuel efficiencies of vehicles, buildings, power plants, and more.
Increase carbon (CO2) sinks, which absorb CO2 - e.g., by planting forests.
Cap emissions release, through national and/or international legislation, alone or in combination with carbon offset options (see below).
Sell or trade carbon offsets or carbon credits. Credits or offsets exchange reductions in CO2 or greenhouse emissions (tree-planting, investment in alternative energy sources, methane capture technologies) for rights to increase CO2 (personally, as for air travel, or industry-wide).
Key urban planning to energy use, e.g., efficient public transportation.
Develop planetary engineering: radical changes in technology (such as building solar shades of dust, sulfates, or microballoons in the stratosphere), culture (population control), or the biosphere (e.g. iron-seeding of the oceans to produce more phytoplankton to absorb more CO2).
Legislate Action: International agreements such as the 2005 Kyoto Protocol (which the US has not yet ratified), or national carbon taxes or caps on emissions. Interestingly, in the U.S., some States and groups of States are taking the lead here.
Set goals of carbon neutrality: in 2007, the Vatican announced plans to become the first carbon-neutral state.
Support developing nations in their efforts to industrialize and increase standards of living without adding to greenhouse gas production.
Every potential solution has costs and benefits which must be carefully considered. Human health, cultural diversity, socioeconomics, and political impacts must be considered and kept in balance. For example, nuclear power involves fewer greenhouse gas emissions, but adds the new problems of longterm radioactive waste transport and storage, danger of radiation exposure to humans and the environment, centralization of power production, and limited supplies of “clean” uranium fuels. Studies of costs and benefits can result in solutions which make effective tradeoffs and therefore progress toward the goal of lowering greenhouse gases and minimizing future global warming.
We have reached the point where we understand how and the extent to which our activities have destabilized the Earth’s atmosphere and reduced and threatened its ecosystem services. Now we need to move one step further, and put our knowledge to work in the form of action.
What will you do to help?
Lesson Summary
The awarding of the 2007 Nobel Peace Prize to the Intergovernmental Panel on Climate Change (IPCC) and former US Vice President Al Gore recognizes the potential impact of global warming on the economic, social, and political welfare of the world.
The greenhouse effect is an ecosystem service which warms the Earth to temperatures which support life.
The greenhouse effect involves water, carbon dioxide, methane, and ozone, which absorb heat that would otherwise be radiated out into space.
Earth’s atmosphere maintains an equilibrium between heat added by sunlight and heat lost by radiation.
The atmosphere of Mars is too thin to hold heat, and that of Venus is so thick that temperatures reach 500oC.
In 2000, the major greenhouse gases were CO2, CH4, and NO; CFCs and H2O contribute, as well.
Global warming refers to an increase in the Earth’s temperature of 0.74°C (1.33°F) within the past 100 years.
Paleoclimatologists document changes in the Earth’s temperature over millions of years which cycle between tropical and ice age extremes – a variation of 10oC.
Greenhouse gases, especially CO2 levels, often correlate w
ith temperature changes.
Deforestation and agriculture – by reducing levels of CO2 uptake – may have initiated warming 8,000 years ago.
Most scientists today agree that fossil fuel combustion, deforestation, and agriculture contribute to greenhouse gases and the greenhouse effect.
Global warming can cause physical changes for the Earth: melting of glaciers and permafrost, changes in precipitation patterns, temperature extremes, warming and acidification of the oceans, and ozone depletion.
Melting of oceans and permafrost can release methane, resulting in a “runaway greenhouse effect.”
Ecological effects may include loss of biodiversity and addition of still more CO2 to the atmosphere.
Socioeconomic threats include crop losses, increased disease, water scarcity, and coastal flooding.
Population growth and socioeconomic factors (especially interference with third world development) can combine to produce political instability and conflict.
Most greenhouse gases are products of fossil fuel combustion, so reduced use, increased efficiency, and alternative fuel development are primary means of prevention of climate change.
CO2 uptake can be increased by eliminating deforestation, reforestation, and green roofs technology.
CK-12 Biology I - Honors Page 89