The Science of Global Warming

Background on Global Warming - Week 2

The greenhouse effect is a very important process that is essential in order for Earth to be inhabitable. Solar radiation enters the Earth's atmosphere and then is either reflected or absorbed by the Earth surface depending upon the albedo of the surface. The reflected part retains spectral properties similar to the incoming radiation, and thus can be reflected out of the atmosphere unimpeded, just as it entered. The absorbed portion of the radiation will become thermal radiation at the temperature of the Earth surface, not at the temperature of the incoming radiation. This is emitted from the surface as infrared radiation, or heat. Because the atmosphere is opaque to infrared radiation it will become trapped and raise the temperature of the Earth. This process keeps Earth at a mean temperature of 10 ^oC, without this process the Earth's mean temperature would be minus 25 ^oC and frozen. Our very existence is dependent upon the greenhouse effect.

You or your students may ask then what the problem is with the greenhouse effect. Under normal conditions the atmosphere naturally traps only a portion of the outgoing infrared radiation. With the increase of greenhouse gases in our atmosphere the proportion of the infrared radiation that is being trapped is increasing. Looking at this trend just with a basic understanding of laws of balance and conservation you should realize that inputs should equal outputs in order to keep equilibrium. If we are seeing less and less output of infrared radiation from our atmosphere, it will pile up and cause warming. Of the greenhouse gases CO ₂ is the most notable since it constitutes about 80% of the greenhouse gas emissions. We emit CO ₂ when we burn fossil fuels to power our communities, transport ourselves, produce the products we use in our lives, or when we burn our forests.

Looking at the greenhouse effect on Venus, with a surface temperature is 400 ^oC, will give us a different view of how increasing the amount of carbon dioxide will affect the amount of warming. The atmosphere on Venus is made up of 90-95% carbon dioxide, which traps almost all of the infrared radiation emitted at the surface of the planet after absorption. The Earth's atmospheric concentration of CO ₂ has made dramatic increases in recent years. Currently, according to NOAA and the Scripps Institution, the Earth's atmosphere has 384ppm (parts per million) of CO ₂ a rise of almost 10ppm in four years from 375ppm in 2002, and a rise of almost 60ppm in the last fifty years from 315ppm in 1958 when the recording started at the Mauna Loa Observatory in Hawaii. You say well fifty years is only blink of time in the history of the planet, is this reason to care?

Scientist have also dug core drills of ice from glaciers around the world and analyzed small bubbles of gas trapped in the snow the year it fell. They can measure how much carbon dioxide was in the Earth's atmosphere at the time. They can also measure the exact temperature by calculating a ratio of isotopic abundance of oxygen-16 and oxygen-18 which can be equated into degrees Celsius. They can even count backward in time year by year by the clear line that separates the years in the ice cores. They have ice core data that depicts temperature and carbon dioxide levels back one thousand years and at no point was the concentration of CO ₂ as high as it is today. Even the medieval warm period is dwarfed by today's measurements of CO ₂ and temperature.

Scientists have been able to ascertain a record of CO ₂ and temperature data from ice cores in Antarctica that dates back 650,000 years and again there is no evidence of a concentration of CO ₂ as high as it is today. There is however a striking trend that when there is more CO ₂ in the atmosphere, the temperature of the Earth increases because there is more infrared radiation trapped within our atmosphere. Current increase of carbon dioxide in the atmosphere contributes to an increase in solar energy input of 0.1W/m ² per year on the Earth's surface. The steady solar energy input is about 340 W/m ². Using these figures, in 100 years the energy input into the Earth's climate system would have increased by 3% at the present time. Scientists have seen a warming trend over the past 140 years by analyzing the annual land, air, and sea surface temperature records that have been recorded since 1860 and have found that the temperature rise has been accelerating recently. Twenty of the twenty one hottest years have happened in the past twenty five years. With the hottest year on record being 2005! Have students perform a math related lesson on graphing global temperature and CO ₂ concentrations and comparing the two graphs.

To understand why there is such an increase in the amount of CO ₂ in the atmosphere you must look at the primary sources of greenhouse gas emissions. In the US, the largest source is from our power plants; approximately 40% come from the burning of fossil fuels for this purpose, and coal accounts for 93% of these emissions. We should look to natural gas as an alternative to coal since natural gas produces half of the CO ₂ emissions for the same amount of energy produced.

The second largest producer of CO ₂ comes from our cars and light trucks, contributing 20% of the total emissions. The US has the lowest standards for gas mileages in the world, with Japan's average nearly double the average American vehicle. For example, using the EPA's 2000 fuel Economy Guide, a 2000 Dodge Durango (produced here in Delaware!) gets twelve miles per gallon in the city and will produce approximately 800 pounds of CO ₂ in a distance of 500 city miles. A 2000 Honda Insight that gets 61 miles to the gallon will only produce approximately 161 ponds of CO ₂ over the same 500 city miles. Imagine how much we could reduce the carbon emissions from our cars if the gas mileage standard was increased. We have the technology to do it; we just need more US consumers to buy vehicles based on fuel economy. Other forms of transportation also play a significant role in carbon emissions. Another 13% of the US carbon emissions come from large, mostly commercial, trucks. The IPCC estimates that aviation also contributes 3.5% of the emissions and could increase to 15% by 2050. And that buildings and structures also give off CO ₂ emissions and contribute to 12% of the US total. The building emissions come mostly from the generation and transmission of electricity accounting for about 70 %, the combustion of natural gas in furnaces and water heaters constitutes about 20 %, and the remaining 10 % comes from fuel oil energy related emissions.

There are numerous other signs of a global warm-up other than just atmospheric data. One very obvious one is the global distribution of ice. You can look anywhere on Earth where there is ice and it is changing. Close to home we can look at glacier national park that was home to around 150 glaciers when it was named in 1910. Today there is fewer that 30 and the projection is that most if not all of the glaciers will be gone within 30 years. The snows of Kilimanjaro have melted more than 80% in the last 100 years. The arctic sea ice has thinned significantly, so much that it has been reduced by 10% in the past 30 years. In permafrost regions in the northern hemisphere, Spring freshwater melt happens nine days earlier than it did 150 years ago and the fall freeze is now ten days later. This thawing of the permafrost has attributed to large areas of ground subsidence in Alaska.

Changes in the salinity and the pH, level of the oceans are another sign of global warming. Salinity of the oceans is changing with a significant increase in the salinity at the equator and a significant decrease at the poles. This is attributed to global warming due to the increase in evaporation tied to the increasing atmospheric temperatures. This evaporation increase at the equator concentrates the salt in the surface of the ocean at those latitudes. The resulting water vapor is then transported to the poles and at the higher latitudes it precipitates and ultimately returns the fresh water to the ocean. The worry is that North Atlantic waters will become too fresh and the ocean conveyor that is driven by the sinking of the saltier water in the northern latitudes could slow down. Analysis of the ice cores has told us that this has happened before in Earth's history. This has caused the North Atlantic to cool and brought drought to other portions of the Northern Hemisphere in the Earth's past.

The changes in pH of the ocean seem inevitable when you think about the fact that over half of the CO ₂ emissions end up in the ocean, a daily dose of approximately 22 million tons! When seawater and CO ₂ combine they form a weak carbonic acid. This is making the ocean more acidic and there is evidence that the lowering of the pH may be harmful to marine life. Many of the animals in the ocean build their shells out of calcium carbonate from seawater. As the pH drops, so will the ability of these organisms to grow. These organisms are the basis of many ocean food chains and support some of the largest organisms on the planet, namely whales.

Many skeptics of global warming will discuss how the Earth will take care of its children, through the guise of the GAIA Hypothesis. This is wishful thinking and the Earth actually is resilient and has been able to offset quite a bit of the anthropogenic effects of our society. It does this by way of natural portions of the planet that are called "sinks". These areas are places that sequester and therefore buffer the effects of the excess carbon input to the system, the identified sinks are the Earth's forests and the oceans, which are now starting to show signs they, are not as efficient at sequestering carbon as they once were. The Kyoto protocol uses this sink concept as a means to achieve emission reduction commitments. For every ton of carbon stored in a sink, the country is allowed to emit an additional ton from the burning of fossil fuel. This concept is flawed since the sequestering of carbon by trees is only temporary. Land use changes, forest fires, and even insect outbreaks can release this carbon into the air. Another interesting concept is that many of these forest plantings by countries to off set their carbon emissions are happening in farmland or boreal regions that would normally covered by ice. By changing the land use of these areas to forests, the albedo is changed so that there is more absorbance of solar radiation. We may find that these newly planted areas are actually adding to the problem more than helping it!

There are serious uncertainties regarding the detailed reactions of the climate system to the increase in greenhouse gases. The problem is not to only identify the significant component of the problem but to understand the feedback between them. The uncertainties stem from a number of different phenomena that are not well understood. These include solar variability, physics of aerosols, and the physics of clouds.

Many of the predictions include temperature increases tied to dates and are not valid since we do not know the actual rate of the increase in the future. What we should be doing is looking how a doubling of carbon dioxide will affect the climate system. Some researchers predict the doubling of atmospheric CO ₂ in a matter of tens of years. For students to understand what this may mean to their existence on this planet the students will engage in a series of activities in which they explore the possible environmental effects.