(35) Global Warming
| 
| Hits: 2438
Climate Tipping Point? Concentration of Carbon Dioxide Tops 400 ppm For First Time in Human History (Video)
Is Your City Doomed? New York, Boston, Mumbai And Bangkok All Face Flood Risks By 2050?
The carbon dioxide connection is finally obvious. Now we need to look ahead and prepare for the inevitable flooding that will happen as this last decade is followed by even quicker sea-level rise.
STABILIZATION AND COMMITMENT
Stabilization
Let’s say that some changes are made around the world and emissions are held steady at some new level. Climate models evaluate what will happen at that new stabilization level.
• The model can evaluate an equilibrium level that indicates what will happen to the climate after an extended period of time.
• The model also can evaluate a transient level that determines what will happen after a specific time period, typically 20 years. Climate models predict the equilibrium temperature increase above preindustrial levels for various greenhouse gas levels.
Just so that we do not have to worry too much about which greenhouse gases are causing the temperature increase, the concentrations are given as carbon dioxide equivalents (CO2, eq). The center line gives the best estimate. The top line gives the highest range, and the bottom line gives the lower range. For instance, if the greenhouse gas level rises to 550 parts per million (ppm) concentration, the best estimate for the average global temperature would be just under 3°C (5.4°F). Notice that this graph is not linear. As the carbon dioxide concentration builds up, there is less infrared radiation left for the added carbon dioxide to absorb.
Results like these from climate models are used to predict the consequences of greenhouse gases added to the atmosphere. Scientists modeling climate have set up several scenarios that enable them to study “what ifs.” These can include different conditions relating to population growth, economic development, and programs implemented in various places to rein in greenhouse gas emissions. The various results are valid only to the extent that the assumptions on which they are based are realistic, and there is not necessarily a single correct answer. By looking at a wide range of possibilities, it is possible to establish a more comprehensive overall picture.
Commitment
The earth’s climate cannot stop on a dime. If all greenhouse gas emissions were suddenly and magically frozen and held at today’s levels without increasing, the earth would continue to warm up because it still would be absorbing more heat than it did in preindustrial times. Even if all greenhouse gas emissions were magically rolled back to preindustrial levels today, the earth still would warm up because of the increased concentration of greenhouse gases that have accumulated in the atmosphere for the past several decades.
For example, if greenhouse gas stabilization occurred in 2100 (for two reasonably likely IPCC scenarios), an additional increase of 0.5°C (0.9°F) still would be expected by 2200. For this case, thermal expansion alone (not even including the effects of snow and ice melting) would lead to an average global sea level increase of 0.3–0.8 m (1–2.6 ft). We have committed ourselves to a certain unavoidable level of global warming, the severity of which will depend on the concentration of greenhouse gases at which the earth stabilizes.
More than half the carbon dioxide added to the atmosphere is removed within about a hundred years. Some smaller fraction, about 20 percent, will take thousands of years before it is removed naturally. For this reason, carbon dioxide levels in the atmosphere will continue to build up even if emission levels are reduced.
Small reductions of carbon dioxide emissions (on the order of 10–30 percent) could be expected to result in a proportional decrease in the growth rate. However, even if all carbon dioxide sources were cut off immediately, the carbon dioxide concentration in the atmosphere would be expected to decline only 40 ppm throughout the twenty-first century. Methane and nitrous oxide have shorter residence times, whereas chlorofluorocarbons (CFCs) have much longer residence times.
How Much and How Soon Will Climate Change?
SENSITIVITY-WHAT HAPPENS IF CARBON DIOXIDE LEVELS DOUBLE?
Svante Ahremius took the fi rst step toward getting a handle on how much an increase in carbon dioxide will affect average global temperatures. He figured that there would be a 5–6°C (7–9°F) increase for every doubling of carbon dioxide levels. Current estimates are lower partly owing to the impact of aerosols in the atmosphere that reflect incoming sunlight before it enters the atmosphere. Current IPCC estimates project that a doubling of carbon dioxide concentration from its present level would increase global average temperatures by 2–4.5°C (3.6–8.1°F). Different researchers have made different assumptions about the details of how the earth’s climate system works, resulting in a range of estimates. The best estimate is that if atmospheric carbon dioxide concentration doubles, the global average temperature would rise by 3°C (5.4°F) and that it is very unlikely to produce an increase of less than 1.5°C (2.7°F).
TEMPERATURE INCREASES FOR VARIOUS SCENARIOS
A critical question on many people’s minds is how quickly the temperature of the earth will rise. Models predict what the temperature will be for different levels of greenhouse gases. Various assumptions are included concerning how quickly greenhouse gases build up, where they are generated, and what is being done to limit emissions. Average global temperature ranges for various scenarios. Each scenario results in different carbon dioxide levels. For instance, for the B1 scenario, the carbon dioxide level stabilizes at 540 ppm. Results are given for each of the following time periods: 2020s, 2050s, 2080s, and 2090s.
Regardless of which scenario is chosen, predicted temperatures are fairly tightly clustered in the forecast for the most recent time period, which is the 2020s. This means that we should not be lulled into a false sense of security if the near-term temperature increases are fairly modest. The real separation between scenarios begins to take effect toward the latter part of the twenty-first century, where more than 2°C (3.6°F) separates the most optimistic and pessimistic scenarios. Not surprisingly, the further out we get, the larger is the spread for a given scenario for a given time.
Higher greenhouse gas levels result in higher temperatures. The differences between high and low carbon dioxide levels will not be as pronounced in the next few decades as they will be toward the end of the twenty-first century. By the 2020s, the average global temperature is expected to increase 0.5–1.0°C (0.9–1.8°F) regardless of what carbon dioxide concentration is established. There are much bigger differences in temperature toward the latter part of the century for different concentrations. For instance, in the 2090s, the worst-case scenario (A1F1, stabilizing at 958 ppm carbon dioxide) results in an average global temperature that is 4°C (7.2°F) higher than the 1980–1999 level. The most optimistic scenario (B1, which has the earth stabilizing at 540 ppm) results in only a 1.5–2°C (2.7–3.6°F ) temperature rise above the 1980–1999 level.
IMPACT AS TEMPERATURES RISE
Climate models have been applied to the glaciers and ice sheets of Greenland3. Results show that if carbon dioxide levels quadrupled from current levels, it would take 270 years for 20 percent of the ice to melt. After 1760 years, 80 percent of the ice would be gone.
HOW QUICKLY WILL TEMPERATURES INCREASE?
Let’s assume for the moment that the world follows a “business as usual” path and continues burning fossil fuels that build up carbon dioxide in the atmosphere (thesis the A2 scenario). By the end of the twenty-fi rst century, temperatures are projected to increase by an average of 3.4°C (6.5°F) above what they were in 2000. Notice that the various models all agree fairly well during the period of instrumental record. Two models have been applied to past climates to reconstruct temperatures derived from proxy measurements. The group of models predicts consistent near-term trends and reasonably similar long-term patterns. The further into the future the models forecast, the greater is the divergence based on differences in underlying assumptions and model details.
HOW RAPIDLY WILL THE SEA LEVEL RISE?
As the average global temperature increases, melting of land-based ice and thermal expansion cause the sea level to rise. By the end of the twenty-first century, continued emission of carbon dioxide from fossil fuel burning is expected to cause the sea level to increase. The various models forecast a rise of somewhere between 0.20 m (0.6 ft) and nearly 0.60 m (2.0 ft). A sea level rise less than 1 m by the end of the century will not cause the worst catastrophic coastal flooding. However, a 3°C (5.4°F) increase in temperature would continue to cause partial melting of the Greenland ice sheet and West Antarctic ice sheets and result in a 4- to 6-m (13–20 ft) increase in sea level within the next few thousand years. If global warming continues unchecked for centuries, complete melting of both the Greenland ice sheet and West Antarctic ice sheets is conceivable, which could result in a sea level rise of 12 m (39 ft). The worse-case scenario (which is not a likely outcome of current climate conditions in the near future), in which there is complete melting of the all land-based ice, would raise sea level by 64 m (211 ft).
OUR WINDOW OF OPPORTUNITY TO ACT
Today, the carbon dioxide equivalent level (CO2, eq) is about 430 ppm and is increasing by about 2 ppm per year. The Stern Review on the Economics of Climate Change (October 30, 2006) suggested that the worst impacts of global warming can be avoided if the carbon dioxide equivalent level is stabilized at between 450 and 550 ppm. This will require emissions to be decreased by 25 percent by 2050 and ultimately stabilized at no more than 80 percent below current levels. According to the report, if no action is taken, carbon dioxide levels could double by 2035. If this were to happen, the overall cost to the world would be at least 5 percent of the world’s economic productivity. However, reduction of global warming would require an undertaking involving no more than around 1 percent of the world’s gross domestic product. The conclusion is that “the benefits of strong and early action far outweigh the economic costs of not acting.”
According to James Hansen1 it takes about 30 years for 50 percent of the equilibrium temperature to be reached and about 250 years for 75 percent of the response and 1000 years for 90 percent of the response to be reached. This implies that climate changes that are already underway from past greenhouse gas emissions are in the pipeline. We have not felt the full brunt of their effects yet and should not be lulled into underestimating their impact based on the present state of the climate.
Waiting to see what will happen can be dangerous because by the time the worst impacts of warming occur, it may be too late to respond. It also means that owing to this “climate inertia,” it will take a long time to see the impact of any reductions of greenhouse gas emissions. Hansen proposed a limit of 450 ppm for carbon dioxide and that it is time now to “move promptly to the next phase of the industrial revolution.”
POPULATIONS MOST AT RISK FROM SEA LEVEL RISE
The most vulnerable and least affluent countries will be affected the most and soonest despite the fact that they have contributed least to global warming. Looking at a satellite image of city lights gives an idea of how close the coastline populations are distributed. About 10 percent of the world’s population lives less than 10 m (33 ft) above sea level, and the majority of the world’s population lives within 10 km (6.2 miles) of sea level. A 5-m (16.4-ft) rise in sea level would cause serious flooding in many cities, including New York, London, Sydney, Vancouver, Mumbai, and Tokyo.
