NCPA | Study #285, Climate Science: Climate Change and Its Impacts

The claim by global warming alarmists that humans are causing dangerous changes in the Earth's climate is based both on several sets of data — temperature measurements, greenhouse gas levels and other phenomena thought to be affected by the climate (such as precipitation) — and General Circulation Models (GCMs) that attempt to predict the Earth's climate. Specifically:

However, the link between human activities and current temperature trends and other environmental impacts is very unclear.

The Data Are Flawed. Our knowledge of present and historic temperature trends is limited by the quality of available data and the way in which it is used. For example, there are major problems with data gathered from direct observation of air temperatures. Air temperature measurements do not accurately represent global patterns because of changes in the location, number, distribution and development surrounding observation sites over the 20th century. For example, city temperatures on warm summer days can be as much as 8° F warmer than the surrounding countryside and annually average about 4.5° F warmer. Over the years, cities have grown so dramatically that many temperature stations are now significantly affected by this urban heat island effect. Cities are heat islands because they have:

In addition to urban observation stations being subject to heat island effects, many other observation stations have either been moved or removed, causing discontinuities in the location of measurements over time. Moreover, observation stations are biased toward mid-latitudes, coastal areas and lower elevations — where most people live and where the urban heat island effect is strongest. Oceans (covering approximately two-thirds of the Earth's surface), high latitudes (the Arctic and Antarctic) and high-altitudes (mountains) are underrepresented.

Arguably, the recorded rise in average global temperatures is due in part to inconsistent data collection, rather than actual warming temperatures. The marked fall in global air temperatures between the early 1960s and the mid-1970s coincided with an increase in the density of observing stations around the globe, whereas the rise over the past 30 years has coincided with a steady decline in station numbers.

Because greenhouse gas concentrations and temperatures were not directly measured for most of human history, much of the data input into the models are estimates based on proxy data, such as the concentration of CO₂ molecules trapped in ice cores. But even when data from direct observations is available, global warming predictions are often based on unrealistic estimates regarding real-world conditions. For example, Hansen, regarded by many as the "father of global warming," recently conceded that CO₂ emissions are now rising 1.0 percent per year, yet computer simulations forming the basis of the Third Assessment Report assumed that emissions would be growing almost twice as fast.³

Furthermore, GCMs predicting substantial global warming in the near future assume far greater per-capita energy use than is currently the case, and far greater future per-capita energy use than most current estimates. By contrast, when models use more realistic estimates of energy use, they produce far less alarming results.

Climate Models Are Limited. To assess future climate trends, climatologists rely upon GCMs that attempt to describe Earth's climate. They include many variables (such as temperature and CO₂ emissions) and make assumptions about how changes in one variable affect others. The many climate models scientists use to generate climate predictions vary widely in which variables they include and in the assumptions they make about how those variables interact. For example, there is considerable scientific debate concerning the overall impact of atmospheric aerosols on the Earth's climate and how the models simulate these effects.

Aerosols are minute particles suspended in the atmosphere. When these particles are sufficiently large, they scatter and absorb sunlight, which can reduce visibility. The resulting haze reddens sunrises and sunsets. Both IPCC and National Assessment projections assume that all atmospheric aerosols have a slight net warming effect. However, more recent scientific data provide a less certain answer.

One study — of which James Hansen is a coauthor — concluded that the warming effect of carbon black aerosols arising from human activities is about twice that used in the IPCC's Third Assessment Report.⁴ But another study suggested that the net effect of sulfate aerosols is to cool the Earth, not warm it.⁵ The net effect of different types of aerosols must first be properly resolved before we can assume a general warming impact on the climate. As the latter study concluded:

"Until [researchers resolve how] the climate system respond[s] [to aerosols], the possibility that most of the warming to date is due to natural variability, as well as the possibility of high climate sensitivity [to the effects of greenhouse gases], must be kept open."

Thus, the extent to which the present warming trend is due to natural factors, human greenhouse gas emissions, and other impacts like the emission of aerosols remains an open question.

Another variable that climate models do not take into account is the effect of changes in solar radiation on the Earth's climate. Over the past 350 years, scientists have discovered cyclical changes in the Earth's climate due to solar activity — such as increases and decreases in solar flares and sun spots. Some researchers have argued that solar variability may be responsible for about 0.45° F of warming between 1900 and 1990 — just under half of the recent warming — and about a third of the total warming since 1500.⁶ This is notable since approximately half of the observed 20th century warming occurred before 1940 and cannot be attributed to human causes. Others have shown that the effect of changes in solar radiation can account for 71 percent of the variation in global surface air temperature from 1880 to 1993.⁷ When changes in solar output are considered in climate simulations, the models predict this warming.⁸ However, it is still not possible to incorporate into a single model all the variables that affect the climate — such as solar variability, changing greenhouse gas concentrations, volcanic eruptions, changes in cloud type and coverage and various pollutants.

Climate Models Do Not Make Accurate Predictions. In addition to problems with the data, computer models are limited by our incomplete understanding of how the Earth's climate responds to a variety of external forces. They are also limited by the speed and capabilities of contemporary computers.⁹ As a result, they do not accurately describe the current climate and have not been able to accurately describe the climate of the past 30 years. For instance, computer models consistently project a rise in global temperatures over the past century that is more than twice as high as the measured increase. As the models cannot explain what has happened in the past, it is fair to question their predictions of future warming. This is particularly true of projections regarding regional changes in temperatures and other climate phenomena. [See the side bar on the U.S. National Assessment.]

The difficulty of reconciling GCM simulations of present-day conditions with real-world observations, and the difficulty of formulating appropriate assumptions about human-caused emissions, led the American Association of State Climatologists (AASC) — a professional organization of regional and state climatologists who use local climate data every day — to conclude in their policy statement on climate change:¹⁰

"Climate predictions have not demonstrated skill in projecting future variability and changes in such important climate conditions as growing season, drought, flood-producing rainfall, heat waves, tropical cyclones and winter storms."

Therefore, relying on climate model simulations to draw conclusions about the future is very risky, since the simulations do not, and perhaps cannot, accurately simulate the present climate.

U. S. National Assessment Relied on Extreme Models

The numerous climate models in use often produce conflicting projections of trends or results at odds with actual observations. The IPCC’s Third Assessment Report compared more than 30 such models. The results from models that predict less warming in the future are generally closer to the observed data.

However, the U.S. National Assessment, published in 2000, used only the HadCM2 (Hadley Centre model) and the CGCM1 (Canadian model) in analyzing the direction and impacts of climate change. It did not include other models from the United States. This is puzzling, since a majority of the models considered in the Third Assessment Report predicts much less warming from a doubling of atmospheric greenhouse gases — as little as 4.5° F. In particular, the Canadian model predicts more future warming under a doubling of atmospheric CO² concentrations than any other model. As Figure II shows:

The National Center for Atmospheric Research (NCAR) model suggests only about 1.8° F of warming globally for the 21st century as a result of increases in greenhouse gases.
The Hadley Centre model produces a warming of 5.4° F.
The Canadian model produces 14.4° F of warming.

Thus, the results of the National Assessment are biased in that the extreme Canadian model was considered, but not the numerous models that suggest only modest warming.

A report submitted for the National Assessment added further doubt to the Hadley Center and Canadian model predictions. The report compared real-world observations with important atmospheric and surface variables used in the Hadley Centre and Canadian models to simulate present-day conditions. Over North America, both models tend to show greater changes in air temperature and precipitation than actual observations. In fact, the Canadian model predicts that the United States should have warmed 2.7° F during the 20th century — 10 times more than the observed increase of about 0.25° F.

For precipitation, the Hadley Centre model and the Canadian model produce the two most extreme projections of changes for the United States — twice the estimates of other models. In particular,
the models’ simulations predict present-day precipitation that differs from actual observations by as much as 100 percent!

Furthermore, both models predict high-pressure systems and winter storms that are much more intense than those we are currently experiencing. They also predict current temperatures for the upper atmosphere (between about 5 km and 20 km in height) that are colder than observed temperatures.

Since these models cannot accurately replicate current climate conditions, it is difficult to place much faith in their projections of future climate in a greenhouse gas-enhanced world. Since the National Assessment relied on these models exclusively, its conclusions are suspect.