"Myths and Facts on Global Warming"

Is global warming happening?

MYTH #1:Winters have been getting colder; for example, the winter of 2003–2004 in the northeastern U.S. was the coldest on record. Therefore global warming can’t be happening.

FACT: Winters have been getting warmer. Independent analyses of measurements by different scientists show that the Earth’s climate has warmed overall over the past century, in all seasons, and in most regions. (The studies are summarized in the Third Assessment Report of the Intergovernmental Panel on Climate Change, 2001.)

The skeptics are doubly misleading when they bill the winter of 2003-2004 as record cold in the northeastern U.S. and imply that it disproves global warming predictions. While that winter was colder than average in the Northeast, it ranked as only the 33rd coldest in that region since records began in 1896 (NCDC, Climate of 2004, 2004). January 2004, when the most severe cold of the season occurred, was the 11th coldest January in the Northeast. No all-time low temperature records were broken in the Northeast that winter (NCDC, Climate of 2004, 2004). Only records for a given day of the year, such as January 16th, were broken, which is nothing unusual; the temperatures were not the coldest that had ever been experienced on any day. This contrasts with the 2003 heat waves in Europe, during which many all-time record high temperatures occurred (see Myth #5). In addition, a single spell of cold weather in one small region is no indication that the Earth’s climate is not warming. “Weather” refers to the atmospheric conditions at a particular time and location, while “climate” refers to the long-term average weather for a location. Scientists say that the Earth’s climate has been warming, since, on average, temperatures have been increasing over a long period of time in most regions. The global average temperature for January 2004 ranked as the fourth warmest on record (NCDC, Climate of 2004, 2004), showing that colder-than-usual weather was limited to a small area. Even with global warming, we will occasionally experience an unusually cold winter somewhere on Earth, since weather varies naturally from year to year and from place to place. But record-breaking cold is expected to become less common as the climate continues to warm, shifting the bounds within which weather fluctuates.

MYTH #2: Satellite measurements of temperature over the past two decades show a much smaller warming a few miles above Earth’s surface than is measured by thermometers at the surface. These observations show that computer climate models are wrong, since they predict that increasing amounts of greenhouse gases should cause slightly greater warming above the surface.

FACT: Recent research has corrected problems that led to underestimates of the warming trend in earlier analyses of satellite data. The early work of Spencer and Christy (1990), as well as subsequent revisions (Christy et al. 2003, and references therein), found little or no warming in the troposphere based on satellite data. Skeptics continue to cite these results. But two recent studies made different corrections to those analyses which, when added together, completely eliminate the discrepancy between climate models and observations. Mears et al. (2003), in a reanalysis of the satellite data, took into account various difficulties in determining a temperature trend from weather satellites. First, since no single satellite has been monitoring temperatures for a long period of time, researchers have had to rely on combining data from different satellites. However, different satellites have different instruments that need to be calibrated precisely against one another. Second, even one satellite may not produce consistent measurements over time, since satellite orbits can drift. After accounting for these factors, Mears et al. found a satellite temperature trend closer in size to the surface trend than in previous studies.

Fu et al. (2004) found another major problem with previous analyses. Satellite data that actually blend temperature readings in the stratosphere (the layer of atmosphere above the troposphere) with those in the troposphere were previously misinterpreted as representing only tropospheric temperatures. A strong cooling of the stratosphere over the same time period partly cancels out the tropospheric warming in the satellite measurements (the averaging of a warming trend and a cooling trend results in little trend).3 Fu et al. isolated the portion of the data that represented just the troposphere, and, accounting also for the corrections by Mears et al., found that the troposphere warmed slightly more than the surface, exactly as models predict. This finding dispels the notion that satellite data disprove the role of greenhouse gases in global warming. Weather balloon measurements may also show a smaller tropospheric warming trend than that observed at the surface. But here again, the interpretation of these measurements suffers from difficulties, such as sparse spatial coverage and inconsistencies among instruments (Fu et al. 2004). So trends derived from balloon data are not reliable.

MYTH #3: Even if Earth has been warming over the past century, it is nothing unusual. For example, temperature reconstructions show that there was a period known as the Medieval Warm Period, roughly from A.D. 1000 to 1400, that was warmer than the 20th century. This means that the global warming we are experiencing now is part of a natural cycle.

FACT: Ten independent scientific studies have all found a large 20th-century warming trend compared to other temperature changes over the past millennium or two. (See Figure 1a.) Some of the studies carefully reconstructed a history of temperatures across the globe over the past 1,000 years or more based on different kinds of records (ice cores, tree rings, historical documents, etc.) The rest of the studies calculated past temperature changes using climate models and an estimate of how climatic factors such as sunlight, volcanic eruptions and greenhouse gases changed over time. Uncertainty exists as to exactly how warm the present is compared to the Medieval Warm Period. Recently, von Storch et al. (2004) argued validly that the method used in some temperature reconstruction studies may underestimate the magnitude of past climate changes; however, the study does not answer the question of how warm the Medieval Warm Period actually was. Further support for the idea of an unusual 20th century warming is provided by a recent temperature reconstruction by Moberg et al. (2005), who consciously avoided the method criticized by von Storch et al. Although they find that the Medieval Warm Period may have been warmer than previously estimated, they “find no evidence for any earlier periods in the last two millennia with warmer conditions than the post-1990 period—in agreement with previous similar studies.” In contrast to these two studies, a number of papers arguing for a very warm Medieval Warm Period contain serious flaws. These include articles by Willie Soon and Sallie Baliunas, and Stephen McIntyre and Ross McKitrick. See the box on “The ‘hockey stick’ controversy.” But what if future research were to produce results drastically different from the scientific studies to date, indicating with a high degree of certainty that the Medieval Warm Period was warmer than the present? That would have very little impact on the scientific case for a human cause of the current warming; whether or not the Medieval Warm Period was warmer provides only one piece of evidence in determining what is causing the current warming. That there were warmer periods in the past caused by natural factors (such as millions of years ago when tropical species flourished in polar regions) is insufficient reason to assume that the current warming also is natural. In fact, a whole body of studies separate from the ones discussed above have focused on the physical factors potentially responsible for changing the climate over the past 150 years, and they indicate with a high degree of confidence that most of the warming over the past 50 years was caused by human-produced greenhouse gases. The current warming is unlikely to be entirely natural and inevitable. Humanity largely controls how much climate change will unfold over the coming centuries.

MYTH #4: The Earth’s climate is self-regulating—natural control mechanisms prevent very large swings. For example, Richard Lindzen claims that the Earth has a planetary “iris,” consisting of tropical cloud cover that expands or contracts to control the amount of heat that escapes from the surface out to space (Lindzen et al. 2001).

FACT: The Earth’s climate actually has been very unstable in the distant past. There have been times, hundreds of millions of years ago, when much of the planet was covered in ice.Tens of millions of years ago, tropical species flourished in polar regions (such as during the era of the dinosaurs). A wealth of scientific evidence indicates that the climate has changed significantly and abruptly (within one or a few decades) numerous times in the past. (For a comprehensive review of the literature on this subject, see the 2002 report by the National Academy of Sciences.) While the climate has been relatively stable since human civilization started around 10,000 years ago, the warming projected to occur over the next century with unrestrained emissions of greenhouse gases is likely to exceed anything seen in the last 10,000 years (IPCC 2001). This warming could cross certain thresholds for abrupt climate change, triggering such dangerous events as a shutdown of the ocean currents that supply nutrients to important fisheries and moderate the climate in Europe and other regions (Sarmiento et al. 2004).

MYTH #5: Recent extreme events such as record heat waves, intense precipitation, massive forest fires and worldwide coral bleaching have no link to climate change.

FACT: These recent events are all consistent with a warming world and can be expected to occur more frequently with global warming. Heat waves and extreme precipitation: Weather fluctuates naturally, resulting at times in extreme events, even in the absence of climate change. However, the increase many critiques by global warming skeptics have focused on the work of one particular group of climate scientists, Dr. Michael Mann and colleagues. Their graph of temperature variations over the past 1,000 years (Mann et al. 1999) is featured prominently in the 2001 Assessment Report of the Intergovernmental Panel on Climate Change (IPCC). in global average temperature over the past century, which constitutes a change in climate, shifts the bounds within which weather fluctuates. This climate change increases the likelihood that high temperature records will be broken and intense precipitation events will occur. (Precipitation becomes more intense because higher temperatures promote increased evaporation and thus higher moisture content in the atmosphere.) In fact, some scientists estimate that human-produced greenhouse gases have already at least doubled the chance of an extreme heat wave like the one that scorched Europe in 2003 (Stott et al. 2004). According to measurements, precipitation appears to have increased in intensity over recent decades, at least in certain regions (IPCC 2001).

During the 2003 heat waves in Europe, many all-time high temperature records were broken. The temperature surpassed 100°F in the United Kingdom for the first time since records began (NCDC, Climate of 2003, 2004). Furthermore, the European heat waves were part of an unusually warm summer overall, which appears to have been the hottest in Europe in at least the last 500 years (Luterbacher et al. 2004).

Forest fires: Global warming is expected to cause, and may already have caused, more severe droughts in many regions. The ongoing drought in the western U.S., possibly the worst in 500 years, may be caused by unusually warm waters in the western Pacific and other effects of global warming (Cook et al. 2004; Hoerling and Kumar 2003). Parched conditions and high temperatures associated with drought create ideal conditions for wildfires to spread. Global warming also lengthens the fire season and promotes outbreaks of tree-killing insects, which create a large amount of dry fuel to stoke forest fires (ACIA 2004). The amount of forest burned annually has been increasing over recent decades in some regions, such as the Arctic (ACIA 2004). Alaska experienced its worst fire year ever in 2004, a year with the warmest and third driest summer on record (NIFC 2004).

Coral bleaching: The incidence of large-scale coral “bleaching” events has increased around the world since 1979, and most evidence indicates that these mass bleachings, which can lead to coral death, are caused by global warming (Hoegh-Guldberg 1999). An increase in water temperature causes coral bleaching, and the average surface ocean temperature in many tropical regions rose by almost one degree C (1.8°F) over the last century. In 1997–98, the largest bleaching event on record damaged 16% of the reefs in the world and killed 1,000-year-old corals (Global Coral Reef Monitoring Network 2004; Hoegh-Guldberg 1999).

The Hockey Stick Controversy

The temperature trend is often known as the “hockey stick” because of its shape: nearly flat for most of the millennium and then a sharp upward turn in the 20th century (see Figure 1b). In their critiques of Mann’s findings, the skeptics claim that they have weakened a primary pillar on which the IPCC’s conclusions about human-produced warming rest. That claim is incorrect. The truth is that other independent studies have found similar temperature trends; the differences that do arise do not affect the overall conclusion that 20th century warming is relatively large and longlasting. Furthermore, how warm the Medieval Warm Period was compared to the present, which is the focal point of the skeptics’ critiques, is only a secondary piece of evidence for what is causing the present warming. The IPCC actually assessed an entirely different body of studies to come to the conclusion that humans caused most of the recent climatic change. Some of the analyses critiquing the work of Mann et al. have serious flaws. For example, a pair of similar papers by Willie Soon and Sallie Baliunas use subjective and unscientific methods to paint a picture of a Medieval Warm Period that was much warmer than the present. A rebuttal was published by thirteen climate scientists (Mann et al. 2003a, b). Soon and Baliunas use incorrect criteria for defining past warm periods. They actually consider a period to be warm even if it was simply unusually wet or dry, rather than basing their judgment strictly on direct indicators of temperature. The authors also consider the entire interval, A.D. 800–1300, to be warm, even though their data indicate that the warmth occurred only in scattered regions at any given time. True global warming occurs when temperatures over large areas increase simultaneously. Also, many scientists have found that the results of their studies were misinterpreted by Soon and Baliunas in a way that bolstered the authors’ thesis (Monastersky 2003).

A paper published in 2003 by Stephen McIntyre and Ross McKitrick, claiming to be an audit of a study by Mann et al. (1998), used data from a corrupted electronic file, arbitrarily omitted 80% of the data before 1600, and replaced certain data with incompatible data, artificially resulting in temperatures during the 1400s that were warmer than during the late 1900s (Rutherford et al., in press, 2005). A paper published in 2005 by McIntyre and McKitrick claims that the hockey stick pattern is an artificial result of the method used by Mann et al. rather than an unbiased compilation of the data. Mann et al. used a version of a method known as principal components analysis, which basically boils a large amount of data down to its main features, or “principal components.” One main problem with McIntyre and McKitrick’s critique is that they misunderstand why Mann et al. used principal components analysis — Mann et al. used it to summarize all the important features of the data, not to extract only the one chief feature. The reason McIntyre and McKitrick’s version of the data doesn’t have a hockey stick pattern is that they display only the first principal component, omitting other critical principal components. In addition, scientists have shown that the hockey stick pattern emerges regardless of what version of principal components analysis is used or even whether principal components analysis is used (Rutherford et al., in press, 2005). Recently, von Storch et al. (2004) argued validly that the method used in Mann et al. and some other temperature reconstruction studies may underestimate the magnitude of past climate changes, particularly the longer-lasting, centuryscale changes such as during the Medieval Warm Period. But this paper provides no definitive answer as to how large the underestimates are (Osborn and Briffa 2004). Also, the temperature reconstruction of Moberg et al. (2005), which avoids the method criticized by von Storch et al., still provides no indication that there were any periods in the past 2,000 years that were warmer than the 1990s.