Forcing Agents Underlying Climate Change: Page 2 of 11
Thank you, Senator McCain.
I will talk about future climate. The most popular climate projection is the "business-as-usual" scenario. It leads to dramatic climate change later in the century. It provides a useful warning of what is possible if greenhouse gases grow more and more rapidly. Four of my colleagues and I recently described an "alternative" scenario for climate change in the 21st century, which we think is a useful complement to the business-as-usual scenario. We assert that a brighter climate future is not only possible, but can be achieved with actions that make good sense independent of global warming. This "alternative scenario" can be explained with the help of my bar chart for the forcing agents that underlie climate change.
These are climate forcings that exist today, compared with 150 years ago. Red is warming, blue cooling. Carbon dioxide causes the largest forcing, 1.4 W/m2. But the forcing by other greenhouse gases — methane, CFCs, ozone and N2O — adds up to as much.
Methane, including its indirect effects, causes a forcing half as large as CO2. Then there are all these aerosols. These are fine particles in the air — solid or liquid particles. Black carbon is soot from diesel engines and coal burning — it causes warming. Sulfates and organic aerosols, from fossil fuel burning, cause cooling. Aerosols also affect the properties of clouds, with a cooling effect, but a large uncertainty.
The question is: How will these forcings change in the future? We could keep the additional climate forcing in the next 50 years as small as 1 watt per square meter by means of two actions: First, we must stop any further net growth of the non-CO2 forcings, several of which are air pollution. Their growth needs to be stopped anyhow, for reasons of human health. Second, CO2 emissions can continue but the emissions rate should be no larger than it is today — preferably declining slowly. The resulting forcing of 1 watt would be expected to cause some climate change, but less than 1 degree Celsius warming in 50 years.
So, how can we stop the growth of these non-CO2 forcings? Black carbon is a product of incomplete combustion — you can see it in the exhaust of diesel trucks. Microscopic soot particles are like tiny sponges, they soak up toxic organics and other aerosols. They are so tiny that when breathed in they penetrate human tissue deeply, some of the smallest enter the blood stream. These particulates cause respiratory and cardiac problems — asthma, acute bronchitis — with tens of thousands of deaths per year in the U.S. Also in Europe where the health costs of particulate air pollution have been estimated at 1.6% of the gross domestic products.
In the developing world the costs are staggering. In India approximately 270,000 children under the age of five die per year from acute respiratory infections caused by air pollution. Most of that pollution arises in household burning of field residue, cow dung, biomass, coal for cooking and heating. There is now a brown cloud of pollution mushrooming from India — you can see it against the Himalayas. There is a similar story for ozone. It is a pollutant that causes tens of billions of dollars of damage per year. We could stop its further growth. Methane also. There are practical steps that could be taken to stop the growth of methane.
The bottom line is that we have only one atmosphere — it's a global atmosphere. My personal opinion is that we need to reduce the pollution that we are putting into it for a number of reasons, especially human health, and in the process we can help prevent the non-CO2 climate forcing from increasing. In the United States, for example, we could reduce diesel emissions and other soot emissions. We might also work with developing countries to help reduce their pollution — one possible long-term solution there would be electrification, a source of clean energy.
I must also address CO2 — it's the hardest part of the problem, but not as hard as it is often made out to be. In 1998 global CO2 emissions declined slightly; in 1999 CO2 emissions declined again; in 2000 I believe that they declined again, but the numbers are not in yet. This is just the trend needed to achieve our alternative scenario, with only moderate climate change. In the near-term, my opinion is that this trend can be maintained via concerted efforts toward increased energy efficiency and increased use of renewable energy sources. On the long-term, most energy experts suggest that we will need a significant increasing contribution from an energy source that produces little or no CO2. In my written testimony I note some possibilities, which include zero-emission coal, nuclear power, the combination of solar energy and hydrogen in fuel cells. Each possibility has pros and cons. I am not recommending policy. R&D is needed. It will be up to the public, via their representatives, to make the choices. My point is that such possibilities exist, so the concept of the alternative scenario, with only moderate climate change, is a viable possibility.
Thank you. I would like to include in the record copies of my final three references. These discuss this topic in more detail but in plain language, which might be helpful.