Theoretical Limits of the Predictability of Global Climate Q12. Are there theoretical limits on the predictability of global climate? Please discuss. A12. Climate forecasts will only be as good as our theoretical understanding, which will never be perfect. Therefore, there will always be (in this sense) a theoretical limit to predictability. Relative Impact of Various Greenhouse Gases Q13. You testified that the earth's greenhouse effect is dominated by water vapor. How much of the greenhouse effect can be attributed to: Q13.1. water vapor? Q13.2. carbon dioxide? Q13.3. methane? Q13.4. CFCs? Q13.5. other greenhouse gases? A13. It depends upon how you calculate it, and how you define “greenhouse effect," but the combined effect of all non-CO2 greenhouse gases is about 96%-98%, which is dominated by water vapor. I haven't seen or calculated the exact contribution of the other minor gases. Q14. How much would a doubling of carbon dioxide increase the overall greenhouse effect? A14. There is no simple answer, again, because it depends upon how you define "greenhouse effect." For example, to calculate the additional radiative forcing at the Earth's surface from doubling CO2, you need to know how the temperature profile of the atmosphere and the temperature of the surface, have changed. Also, the influence of feedbacks can then alter the greenhouse effect. The IPCC puts all of this in terms of "radiative forcing," which seems to be the most accurate way to quantify the effect. Compared to an average infrared cooling rate of 250 W/m2, the doubling of CO2 is thought to cause a reduction of the cooling rate by about 4 W/m2, which is a 1.6% effect. But again, this value depends upon some assumed change in tropospheric and stratospheric temperatures, which could Satellite Temperature Data vs. Surface Measurements Q15. In their March 13, 1997, Nature article, Hurrell and Trenberth argue that the microwave sounder unit (MSU) data, while useful for many purposes, are poorly suited for gauging long-term surface temperature trends. MSUs monitor the globe more thoroughly than surface reports, which are concentrated over land and approximated over oceans. However, each MSU lasts only a few years, to be replaced by another deployment on a different satellite. According to the NCAR scientists, the transitions between satellites may be producing spurious temperature drops that mask an actual rise in global readings. "The surface and MSU records measure different physical quantities," write Hurrell and Trenberth, "so that decadal trends should not be expected to be the same." However, they add, "unreconciled discrepancies among the different records remain.” 'To study the matter further, Hurrell and Trenberth focused on the tropics between 20 degrees N and S, where “noise” from short-term weather variations is lower than it is in temperate and polar zones. They compared simultaneous MSU records to each other, to sea-surface temperatures (SSTs), and to air temperatures simulated by an NCAR climate model using SSTs. And they found that most of the difference between MSU and surface trends could be explained by two significant drops in MSU data for 1981 and 1991, years when satellite transitions took place. Q15.1. Please provide for the record your views on the reliability of these data. Q15.2. Please comment on the validity of the following statement: "Satellite observations were initially interpreted as showing a slight cooling, but more recent analyses accounting for natural, short-term fluctuations imply warming." A15.2. This is a scientifically irresponsible statement. It implies that the satellite record was somehow "corrected," when in fact all the authors did was to assume that the satellite should be measuring the same trends as the surface data (which have their own measurement problems). Their use of a climate model forced by observed sea surface temperatures during the period of satellite record is known to produce tropospheric temperature variations which will vary in lock step with the surface, due to incomplete physics. For instance, ozone depletion and two of the largest volcanic eruptions of the century were not included in those model runs, all of which can affect the vertical profile of temperature. Is Climate Change Underway? A16. Did the 1995 report of the Intergovernmental Panel on Climate Change (IPCC) conclude that climate change is already underway, and if so, what specific climate change? Q16. I am assuming you mean climate change due to mankind's activities. The executive summary of that report states that all of the available evidence, when considered together, suggests some small influence of mankind. However, the main body of the report suggests that we can not determine with any kind of confidence whether any of the observed climate change is due to man's activity. FOLLOW-UP QUESTIONS FROM DEMOCRATIC MEMBERS Validity of Ground Level Temperature Measurements and Climate Models Q1. Your testimony suggests that Global Circulation Models do not treat water vapor realistically and hence, are fatally flawed in representing the feedback effects associated with greenhouse gas emissions. Your testimony also refers to research that calls into question the validity of ground level temperature measurements and their representativeness with respect to global temperature trends. Yet, the IPCC consensus was strongly impacted by the striking ability of GCMs to reproduce historical ground level temperature trends when aerosols are included. Since your work calls into question both the data and the models, how do you explain this agreement? Al. First, I would not say that the models are "fatally flawed" on the water vapor feedback issue. However, if they are correct, it would be largely by accident. This is because we don't know how precipitation efficiency will change in the presence of increasing CO2, a process which controls water vapor feedback. The model vs. observations agreement was largely the result of a work by Santer et al. (Nature, 4 July 1996), when a very selective time period was addressed, from 1964 to the late 1987. P. Michaels (Nature), pointed out that these results break down when additional years are added. Also, the agreement that was found was mostly in the Southern Hemisphere, where the observational data are very poor, and could very well have been fortuitous. Water Vapor Feedback Loop Q2. A2. Your testimony questions the ability of water vapor to act as a feedback loop to amplify the direct effects of greenhouse gases such as CO2. How do you explain the enhanced temperatures evident in the geologic record (e.g. during the Cretaceous period) when CO2 levels were high? The issue here is cause and effect. If indeed the CO2 and temperature levels were significantly higher in the geologic past, then the elevated CO2 could be due to the higher temperatures, instead of vice versa. Biologic activity might explain this (e.g. during warmer conditions, increased soil respiration causes an increase in CO2 that more than compensates for the increased uptake of CO2 by enhanced plant growth). I do not have much confidence in what global climatic conditions were in the geologic past, since the evidence is so indirect. (This field of science barely qualifies as science in a strict sense, since there were no observations, and no way to validate or disprove the theories.) Another possibility is that the solar output of the sun was higher then (I believe there is positive water vapor feedback when the radiative forcing is due to sunlight). 46-495-14 |