Research Features

Forcing Agents Underlying Climate Change: Page 4 of 11

Submitted Testimony: 1. Preface.

Mr. Chairman and Members of the Committee:

I appreciate the opportunity to clarify the paper I co-authored with four other scientists on climate change in the 21st century, published in Proceedings of the National Academy of Sciences (1). In that paper, we define an "alternative scenario" for the forcing agents that cause climate change. The alternative scenario gives equal emphasis to reducing air pollution and to a continued slow downtrend in CO2 emissions. This scenario produces only a moderate climate change in the next 50 years. We suggest that the climate forcings in this scenario can be achieved via pragmatic actions that make good sense for a variety of reasons. Collateral benefits include improvements in human health, agricultural productivity, and greater energy self-sufficiency. Our alternative scenario differs markedly from the "business as usual" scenarios of the Intergovernmental Panel on Climate Change (IPCC), which have received the greatest attention among the plethora of IPCC scenarios. However, I emphasize that our paper is not a criticism of IPCC. The IPCC reports (2), produced by hundreds of outstanding scientists, provide an invaluable assessment of the status of scientific understanding of climate change.

Although our research has relevance to public issues, it is not our job to suggest policies. Our objective is to provide scientific information that the public and their representatives can use to help choose wise policies. Thus our aim is to provide relevant information on the forcing agents that drive climate change that is as quantitative and as clear as the data permit.

2. Introduction: Basic Concepts.

The Earth's climate fluctuates from year to year and century to century, just as the weather fluctuates from day to day. It is a chaotic

Figure 1
Fig. 1: Climate forcing during the Ice Age 20,000 years ago relative to the current interglacial period. This forcing of -6.6 ± 1.5 W/m2 and the 5°C cooling of the Ice Age imply a climate sensitivity of 0.75°C per 1 W/m2.

system, so changes occur without any forcing, but the chaotic changes are limited in magnitude. The climate also responds to forcings. If the sun brightens, a natural forcing, the Earth becomes warmer. If a large volcano spews aerosols into the stratosphere, these small particles reflect sunlight away and the Earth tends to cool. There are also human-made forcings.

We measure forcings in watts per square meter (W/m2). For example, all the human-made greenhouse gases now cause a forcing of more than 2 W/m2. It is as if we have placed two miniature Christmas tree bulbs over every square meter of the Earth's surface. That is equivalent to increasing the brightness of the sun by about 1 percent.

We understand reasonably well how sensitive the Earth's climate is to a forcing. Our most reliable measure comes from the history of the Earth. We can compare the current warm period, which has existed several thousand years, to the previous ice age, about 20,000 years ago (3, 4, 5). We know the composition of the atmosphere during the ice age from bubbles of air that were trapped as the ice sheets on Greenland and Antarctica built up from snowfall. There was less carbon dioxide (CO2) and less methane (CH4), but more dust in the air. The surface was different then, with ice sheets covering Canada and parts of Europe, different distributions of vegetation, even the coast-lines differed because sea level was 300 feet lower. These changes, as summarized in Figure 1, caused a negative climate forcing of about 6-1/2 W/m2. That forcing maintained a planet that was 5° C colder than today. This empirical information implies that climate sensitivity is about 3/4 ° C per watt of forcing. Climate models have about the same sensitivity, which provides encouraging agreement between the real world and the complex computer models that we use to predict how climate may change in the future.

There is another important concept to understand. The climate cannot respond immediately to a forcing, because of the long time needed to warm the ocean. It takes a few decades to achieve just half of the equilibrium climate response to a forcing. Even in 100 years the response may be only 60-90 percent complete (5). This long response time complicates the problem for policy-makers. It means that we can put into the pipeline climate change that will only emerge during the lives of our children and grandchildren. Therefore we must be alert to detect and understand climate change early on, so that the most appropriate policies can be adopted.

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