The case for attributing the recent global warming to human activities rests on the following undisputed scientific facts:
Carbon dioxide (CO2) is a greenhouse gas that warms the atmosphere. Since pre-industrial times, atmospheric CO2 concentrations have increased from about 280 parts per million (ppm) to over 380 ppm. Current concentrations of CO2 and other greenhouse gases are unprecedented in at least the last 650,000 years, based on records from gas bubbles trapped in polar ice. Independent measurements demonstrate that the increased CO2 in the atmosphere comes from burning fossil fuels and forests. The isotopic composition of carbon from these sources contains unique “fingerprint.” Since pre-industrial times, global average temperatures have increased by about 0.7ºC, with about half of the warming occurring over the past few decades. The only quantitative and internally consistent explanation for the recent global warming includes the intensified greenhouse effect caused by the increase in CO2 and other greenhouse gases.
Natural processes emit large quantities of CO2 into the atmosphere, but they also remove it—at nearly identical rates. This balance maintained the concentration of CO2 at a stable level for thousands of years prior to the Industrial Revolution. In the case of global warming, the question is: What is causing the increase in CO2 concentrations? The answer turns out to be incontrovertible. The isotopic composition of carbon in atmospheric CO2 provides a unique “fingerprint” that tells scientists that the lion’s share of the additional CO2 accumulating in the atmosphere is from the burning of fossil fuels.
The global climate has fluctuated considerably over the Earth’s history, either for unknown reasons or because of “internal variability” in the climate system. We do not know enough about the climate system to attribute the present global warming to any specific cause.
It is true that the Earth’s climate has exhibited wide swings over geologic time due to natural processes. However, scientists have reasonable qualitative explanations for most of the significant variations in climate over geologic time; they can be largely attributed to specific processes, not to unknown internal oscillations. Many of the major climatic changes can be traced to changes in the Earth’s orbit around the sun. Others can be linked to specific events (such as the impact of a comet or meteorite or the assembly or breakup of supercontinents) that led to large changes in the concentration of atmospheric greenhouse gases. For more recent times (the past millennium), scientists have been able to quantitatively attribute the major temperature fluctuations to changes in solar activity, volcanic eruptions, and human-produced greenhouse gases and particulate pollution. These natural processes can not explain the current warming.
The current warming trend is analogous to the Medieval Warming Period (MWP). Since the MWP was obviously a natural event, the current warming is also likely caused by natural processes.
The Medieval Warm Period (MWP) refers to a relatively warm period lasting from about the 10th to the 14th century. However, the initial evidence for the Medieval Warm Period (MWP) was largely based on data gathered from Europe, and more recent analyses indicate that the Medieval Warm Period (MWP) was not a global phenomenon.
A number of reconstructions of millennium-scale global temperatures have indicated that the maximum globally averaged temperature during the Medieval Warm Period (MWP) was not as extreme as present-day temperatures and that the warming was regional rather than global. This conclusion in the 2001 assessment of the Intergovernmental Panel on Climate Change: “The…’Medieval Warm Period’ appear(s) to have limited utility in describing trends in hemispheric or global mean temperature changes in past centuries.”
Observations of melting high altitude glaciers are perhaps even more telling. Andean glaciers that have been intact for more than 5,000 years are now rapidly melting. If the Medieval Warm Period (MWP) was truly global, these glaciers would not have survived.
More generally, it is a logical fallacy to argue that because the climate has changed in the past due to natural causes, the current warming trend must also be due to natural causes. The debate over the magnitude and causes of earlier climate change such as the Medieval Warm Period (MWP) is of scientific interest, but it does not invalidate the considerable direct scientific evidence that human-produced greenhouse gases have been causing the Earth to warm recently.
The Earth’s climate for the past 2 million years has been characterized by ice ages lasting close to 100,000 years, punctuated by relatively short (10,000- to 30,000-year) warm periods or “interglacials.” The swing from glacial to interglacial is caused by changes in the Earth’s orbit around the sun amplified by natural feedbacks involving greenhouse gases.
The Earth entered the present interglacial about 10,000 years ago. All things being equal (i.e., in the absence of a large human-produced source of CO2) it is highly likely that the Earth will swing back into a glacial period or ice age. But this will not occur for thousands of years.
As early as the 19th century, scientists recognized that greenhouse gases warm the planet, and that increases in atmospheric carbon dioxide could lead to global warming on time scales of decades to centuries—much shorter than the fluctuations related to ice ages and interglacials. Around the same time, global temperatures began to increase and scientists became increasingly concerned that humans were interfering with the climate.
In the 1950s the upward trend in global temperatures unexpectedly halted and temperatures declined somewhat. This led some scientists to become concerned about global cooling and, in turn, to headlines in the popular press about an imminent ice age. What the skeptics fail to admit is that within the scientific literature—as opposed to the popular press—global warming remained a serious concern. Many scientists of the time argued that whatever the cause of the cooling, natural or otherwise, it would be eventually overshadowed by the warming effect of carbon dioxide. In 1979, the National Academy of Sciences warned that a doubling of carbon dioxide would increase global temperatures by 1.5 to 4.5oC and shortly thereafter a resumption of the upward trend in temperatures was detected.
Over the past quarter century, scientific research on global climate change has intensified, and programs on an international scale have been organized. More and more data are included in computer models that are capable of recreating past trends and more precisely predicting future scenarios. We now know that the mid-20th century pause in global warming was caused by pollution from burning coal, which produced tiny particles or aerosols that blocked the energy from the sun. As aerosol emissions were controlled but greenhouse gas pollution continued to increase, the cooling effect of the aerosols was overwhelmed by the greenhouse gases, and global warming resumed.
It is of course true that, in a complex system like climate, it is virtually impossible to prove a negative; i.e., that natural processes are not causing the current warming. What we can do is eliminate every possible natural explanation that can be posited.
Thermodynamics tells us that the warming of the Earth’s lower atmosphere must arise from one or more processes that supply excess heat to the lower atmosphere. Besides the greenhouse effect, the viable processes are (1) increased output from the sun; (2) increased absorption of heat from the sun due to a change in the Earth’s planetary reflectivity or “albedo”; and (3) an internal variation in the climate system that transfers heat from one part of the Earth to the atmosphere. Direct observations confirm that none of these explains the observed warming over the latter half of the 20th century. For example there has been no appreciable change in solar output over the past two decades.
Satellite data reveal that the Earth’s reflectivity increased (causing cooling instead of warming) in the ’60’s, ‘70s, and early ‘80s and has decreased modestly since. The overall warming from the recent decrease in reflectivity is also small compared to the greenhouse warming.
In the case of internal variations, the ocean is the only viable reservoir of internal heat that could have caused the atmosphere to warm on decadal time-scales. However, observations show that the heat content of the ocean has increased instead of decreased over the past few decades. This indicates that the atmosphere has been a source of heat to the ocean rather than vice versa. Moreover, the amount of heat increase in the ocean is consistent with what is needed to balance the Earth’s energy budget given the excess heating from the enhanced greenhouse effect and the amount of excess heat observed to be stored in the atmosphere. In other words, the amount of heat stored in the ocean over recent years matches the amount of heat that models predict should be trapped on Earth due to the increase in greenhouse gases.
After carefully analyzing the evidence and considering all relevant factors, we can confidently draw the conclusion that
• The Medieval Warm Period does not represent an analogy to the warming of the late 20th century, for which scientists have independent evidence of human causation, and the evidence strongly suggests that the MWP was a regional, rather than a global phenomenon.
• Our understanding of the climate system is sufficient to provide qualitative models for most global or hemispheric climatic variations over geologic history and quantitative models for variations over the past millennium.
• The Earth’s climate may return to ice age conditions in thousands of years, but this does not preclude devastating effects from global warming over the next few centuries.