Suvi Flagan ESE/Ge 148a A Review of Soden et al: Global Cooling After the Eruption of Mount Pinatubo: A Test of Climate Feedback by Water Vapor. By: BJ Soden, RT Wetherald, GL Stenchikov, and A Robock. 26 April 2002 Science, Volume 296, pgs. 727-730 The feedbacks from water vapor are some of the most difficult to quantify, or were until this paper by Soden and his fellow researchers at Princeton University and Rutgers University in New Jersey. (1) They found a way to determine the magnitude of the large positive water feedbacks assumed by most climate models. Most models assume a constant relative humidity in the atmosphere that corresponds to global temperature changes, such that half of the observed feedbacks are assumed to be caused by water vapor. Soden and colleagues confirmed that this is an accurate assumption. They determined that 60% of the observed cooling after the eruption of Mount Pinatubo was due to a drying of the lower troposphere. To determine this value for water vapor feedbacks, Soden et al. compared satellite observations to GCM predictions in the years 1991 to 1995. In June 1991, the eruption of Mount Pinatubo in the Philippines released large amount of sulfur and
other volcanic aerosols into the atmosphere. Satellite observations show a decrease in global temperatures immediately following this eruption. Soden et al. used this cooling, and recent data about the effects of the strong El Niño experienced during 1992 (2), to determine what percentage of this cooling was due to water vapor and what part was due to other climate anomalies and normal climate effects experienced in the same time period. To begin, they show that their models are accurate in reproducing longwave (terrestrial) anomalies and shortwave (solar) anomalies. These longwave anomalies are used as a proxy for water vapor feedbacks in many climate models. By artificially removing the longwave component of the feedback loop, the climate model has a substantially lowered water vapor feedback that does not show the perturbations expected over the 5 year record in longer studies. This was used to show that the climate effects after Mount Pinatubo s eruption can be studied on a small timescale. Second, they compare globally averaged temperatures and total column water vapor to show a 6% per year drying of the lower troposphere. Figure 1:
A comparison of the two graphs in Figure 1 shows a strong correlation between temperature and water vapor in the atmosphere. Soden et al. found a 6% per year drying of the atmosphere that corresponds to the rate that saturation water vapor pressure decreases with temperature in the lower troposphere. This correlation between saturation water vapor and temperature implies a constant relative humidity. This seems to be implied by the constant drying of the atmosphere observed in Figure 2. However, they admit some large uncertainties in this method, so they follow it with a third analysis of satellite observations and model predictions. Soden and colleagues looked at the 6.7 µm band in atmospheric records, where climate is very sensitive to changes in relative humidity in the upper troposphere. Under clear skies, the 6.7 µm band is sensitive to water vapor absorption, such that if water vapor in the upper troposphere decreases by conserving relative humidity as the atmosphere cools, the temperature record at 6.7 µm should show only a small perturbation.
Figure 2: In Figure 2, this small perturbation in T 6.7µm is shown both by satellite observations and GCM predictions. Three models were used in this analysis, the first with water vapor feedbacks, the second with a constant relative humidity, and the third with no drying (water vapor feedbacks removed). The first two models make predictions that fit very well with observations. The GCM with no drying does not reproduce the same record. This shows that the atmospheric water content decreases in accordance to radiationinduced cooling observed after the eruption of Mount Pinatubo. Last, after showing that their models accurately predict the cooling and drying of the atmosphere in response to Mount Pinatubo s eruption, they look at the temperature effects of water vapor with and without the El Niño oscillation signal removed. Again, their data fits the observed trends fairly well. They show that the model predicted average global cooling of 0.31 K from June 1991 to December 1995 is very close to the observed cooling of 0.30 K (0.33 ± 0.03 K) with the El Niño signal removed; while the model predicted average without water vapor feedbacks is only 0.19 K. Thus, the feedback from water vapor amplifies the magnitude of global cooling by ~60%. This
agrees with climate models in response to the temperature amplification due to a doubling of CO 2, and with calculations using a constant relative humidity. They show that assuming a constant relative water vapor feedback of roughly half is an accurate estimate of the true effects of water vapor on the climate system. Some uncertainty in their models remains. Soden et al. assumed clear skies in their calculations of water vapor absorption in the 6.7µm band. The majority of the time, clouds occupy a fraction of the sky, adding a possible substantial feedback to the equation. Some of the model predicted and observed cooling could be due to the clouds in the sky. Also, the behavior of sulfur aerosols emitted by volcanoes is not completely understood. Some of the observed climate changes could be changes attributed to aerosol feedbacks, not water vapor. (3) In climate models, the water vapor feedback is one of the larger uncertainties. However, its effects on the climate system are essential to understanding the effects of a doubling of carbon dioxide, and therefore on making predictions of the future climate of Earth. Most models assume a constant relative humidity in the lower troposphere for several reasons. First, the lower troposphere is very well mixed, such that the perturbations in the natural system from the eruption of a volcano very quickly are dispersed evenly throughout that mixed layer. Second and more importantly, saturation water vapor pressure is known to decrease at a constant rate with changes in temperature. Because of this characteristic of constant water vapor pressure, a nearly constant relative humidity change in water vapor mass is assumed.
As mentioned earlier, understanding the water vapor feedbacks is essential to make accurate predictions about the future climate systems. The IPCC recently published their Technical Summary for Policymakers in which they bring up the fact that most of the atmosphere is not saturated with water vapor. (4) Condensation, evaporation, and transport by winds cause water vapor concentrations to be even more ambiguous on global scales. The fact that the saturation water vapor pressure decreases with temperature does not mean that water vapor pressure decreases everywhere the same with decreasing temperatures. In addition, clouds carry an even greater uncertainty, and where there are particles in the air and high water vapor pressures clouds can be found. More knowledge about the mechanisms of water vapor to climate change could lead to increased understanding about the mechanisms of clouds. As the temperatures increase, water vapor increases, increasing the chance for cloud formation. Since the IPCC is interested in making predictions about the future, knowing that all models use the same assumptions and values for relative humidity increases the credibility of their results. A review by Anthony Del Genio, published in the same issue of Science as this article, examines the water vapor feedback observed by Soden et al. with respect to natural perturbations and its effect on global warming. Water vapor absorbs radiation from the surface and re-emits some of it to space. The global warming feedback as understood by most scientists is based on the fact that cooler air radiates less than warmer air: if water vapor increases and is distributed to higher, colder altitudes, less heat is radiated out to space and thus the climate warms. (3) He concludes that,
though some uncertainty remains after the paper from Soden et al., it is confirmed that a large increase in particulate matter in the atmosphere (say carbon dioxide or methane) will lead to an even larger increase in temperatures due to water vapor feedbacks. This paper by Soden and colleagues reduces the uncertainty in water vapor feedbacks. Though they do not have an exact value for the forcing due to water vapor, they confirm that most climate models are accurate in assuming a constant relative humidity in the lower troposphere. References 1. Soden BJ, Wetherald RT, Stenchikov GL, Robock A. Global Cooling After the Eruption of Mount Pinatubo: A Test of Climate Feedback by Water Vapor. Science 296: 727 (2002). 2. Santer BD, et al. Accounting for the effects of volcanoes and ENSO in comparisons of modeled and observed temperature trends. Journal of Geophysical Research Atmospheres 106: 28033 (2001). 3. Del Genio AD. The Dust Settles on Water Vapor Feedback. Science 296: 665 (2002). 4. Albritton DL, Meira Filho LG. Technical Summary: A report accepted by Working Group I of the IPCC but not approved in detail. IPCC (2001). 5. Robock A. Volcanic Eruptions and Climate. Reviews of Geophysics 38: 191 (2000).