Global warming: a sceptic's perspective

The scientific community does not question that global mean surface temperatures have increased during the 20th century, and that they will continue to increase significantly during the next century, as a result of anticipated increases in greenhouse gas concentrations. Yet the community also admits that the magnitude of the change is uncertain, for instance because of the uncertainties about the indirect radiative forcing of aerosols.

A few people have expressed scepticism towards this generally accepted view, and some of the arguments are listed below. We have provided 'mainstream' responses to each of the six arguments.

• Much of the warming occurred in the first half of the 20th century, whereas greenhouse gas concentrations have increased mostly during the second half. And the early warming was merely a natural rebound to normal conditions, after the Little Ice Age which ended around 1850.
• The response to this argument is that the lack of warming between 1945-1975 was probably due to the competing cooling effect of (mostly manmade) aerosols. Detailed GCM simulations incorporating the observed changes in concentrations of greenhouse gases, stratospheric ozone, and aerosol, as well as in solar activity, replicate the observed temperature changes during the last few decades well. The argument ignores the accelerated warming since 1975.
• 
  Satellite-based estimates (MSU) suggest that the lower tropospheric temperature has not increased since 1979, in fact it may have slightly decreased. During this period the global mean station temperature increased, but many stations also became increasingly surrounded by urban developments, i.e. the observed heating of 0.2K since 1979 may be entirely due to urban heating.
• The response to this argument is that very recently a bias has been discovered in the MSU data, due to orbital decay of the satellite, and correction for this bias brings the MSU data in closer agreement with the surface observations (1). Also measurements over sea and from surface stations far from cities indicate a global warming trend.
• 
  The observed temperature variations during the last century, in fact those during the last six centuries (including the Medieval Optimum and the Little Ice Age) can be entirely explained by inherent, internal variability of the global climate, i.e. not by any external forcing mechanism (2).
• The response to this argument is that several independent GCM simulations show that the rapid warming during the last two decades cannot be explained by natural climate variability.
• 
  Evidence from Mars and Venus suggest that global warming from doubled carbon dioxide in the atmosphere is unlikely to exceed 0.5 K (3). The atmospheres of these planets consist almost exclusively of CO₂ (Table 1.2). Venus has an atmosphere containing CO₂ at a pressure of 88 bars, i.e. 88 times our atmosphere’s total pressure at sea level. Such an amount of CO₂ causes greenhouse warming by 500 K there. On the other hand, the mere 0.006 bars of CO₂ on Mars cause warming by 5.5K. These figures can be plotted on a graph of the logarithm of the pressure against the logarithm of the warming. The straight line between these two points can be extrapolated to find the warming effect of 600 ppm of CO₂ in the Earth's atmosphere, i.e. 0.0006 bars. The answer is 0.47 K. This is only one of eight observed relationships between radiation and surface temperatures, each indicating only small effects from doubled CO₂ (3).
• The response to the extrapolation of Mars and Venus data is that simple one-dimensional radiative transfer models indicate that CO₂ doubling causes warming of 3-5 K. The 1895 calculations by Svante Arrhenius of the effect of an increased amount of atmospheric CO₂ are more accurate than those presented in this argument (4). In reality warming may be less because of some negative feedbacks.
• 
  The overall steadiness of climates during the history of the Earth points to over-riding negative feedbacks which inhibit drastic alteration. The Gaia hypothesis suggests that the biosphere regulates the atmospheric temperature (Section 1.2). Biospheric responses to the current climate change, as well as the potentially large effect of changes in cloudiness, cannot be simulated by GCMs. Temperatures slightly warmer than the current ones where found on Earth during the Carboniferous era, when the Sun's luminosity was 2.5% less than today.
• The response is that the Pleistocene has experienced fairly large temperature oscillations, resulting in several Ice Ages. If there are two quasi-stable climatic states, there may be more. As the concentration of greenhouse gases is reaching levels that have never been experienced during the Pleistocene, it is possible that we are moving towards an unprecedented quasi-equilibrium in climate.
• 
  The Little Ice Age coincided with a quiet Sun, i.e. sunspots were virtually absent. The higher the sunspot activity, the higher the solar irradiance tends to be. Recent satellite measurements of solar brightness show an increase from the previous cycle of sunspot activity to the current one, which may explain the observed warming during the last two decades (5).
• The response to this is that sunspot activity does have an effect on climate, although it appears small compared to the effect greenhouse gases. And the observed temperature trend during the 20^th century cannot be solely explained by variations in solar activity.

It will be interesting to see comments on Idso's paper (3) by representatives of the mainstream view, that the enhanced greenhouse effect is much more serious.

1. Wentz, F. J. and M. Schabel 1998. Effects of orbital decay on satellite- derived lower-tropospheric temperature trends. Nature, 394, 661.
2. Mann, M.E., R.S. Bradley and M.K. Hughes 1998. Global-scale temperature patterns and climate forcing over the past six centuries. Nature, 392, 779-803.
3. Idso, S.B. 1998. CO₂-induced global warming: a skeptic’s view of potential climate change. Climate Research, 10, 69-82.
4. Weir, A. 1995. 100 years ago: Arrhenius moonlights on the greenhouse effect. Aust. Meteor. Ocean Soc. Bull., 8, 109-10
5. Willson, R.C. 1997. Total solar irradiance trend during Solar Cycles 21 and 22. Science, 277, 1963-5.