Page 360 - Geosystems An Introduction to Physical Geography 4th Canadian Edition

324 part II The Water, Weather, and Climate Systems
climatic trends. Some feedback is negative, acting to slow the warming or cooling trend. For example, increasing CO2 stored in the atmosphere increases global warm- ing, which increases the amount of water vapour present in the atmosphere (warm air masses absorb more mois- ture). Greater atmospheric moisture in a warmer climate generally leads to greater precipitation. With increasing rainfall comes an increase in the breakdown of exposed rock on Earth’s surface by chemical weathering processes (discussed in Chapter 14). This process occurs over long time periods as CO2 in the atmosphere dissolves in rain- water to form a weak acid (carbonic acid, H2CO3), which then falls to the ground and works to chemically break down rocks. The outputs of this weathering, consisting of chemical compounds dissolved in water, are carried into rivers and eventually oceans, where they are stored for hundreds of thousands of years in seawater, marine sedi- ments, or corals. The result is that, over long time peri- ods, CO2 is removed from the atmosphere and transferred to the ocean carbon sink. This creates a negative climate feedback because the overall effect is to reduce the global warming trend.
This so-called CO2–weathering feedback provides a natural buffer to climatic change. For example, if out- gassing increases and the level of atmospheric CO2 rises, global warming will occur. The subsequent increase in precipitation and enhancement of chemical weathering then acts to buffer warming by removing CO2 from the atmosphere and transferring it to the oceans. If a change in orbital cycles or oceanic circulation triggered global cooling, then the reduction of precipitation and chemi- cal weathering would leave more CO2 in the atmosphere, where it would work to increase temperatures. Through- out Earth’s history, this natural buffer has helped prevent Earth’s climate from becoming too warm or too cold. The rapid pace of present climate change, however, is evi- dently beyond the ability of natural systems to moderate.
Evidence for Present Climate Change
In previous chapters, we discussed many aspects of contemporary climate change as we explored Earth’s at- mosphere and hydrosphere. In subsequent chapters, we examine the effects of climate change on Earth’s litho- sphere and biosphere. The task of the present chapter is to review and consolidate the evidence for climate change, revisiting some issues and introducing others.
The evidence for climate change comes from a va- riety of measurements showing global trends over the past century, and especially over the past two decades. Data gathered from weather stations, orbiting satellites, weather balloons, ships, buoys, and aircraft confirm the presence of a number of key indicators. New evidence and climate change reports are emerging regularly; a trusted source for the latest updates on climate change science is the Intergovernmental Panel on Climate Change (IPCC),
operating since 1988 and issuing its Fifth Assessment Report between September 2013 and April 2014 (see www .ipcc.ch/). Also see Canada’s Action on Climate Change, at www.climatechange.gc.ca, and the U.S. Global Change Re- search Program, at www.globalchange.gov/home.
In this section, we present and discuss the measurable indicators that unequivocally show climatic warming:
• Increasing temperatures over land and ocean surfaces, and in the troposphere
• Increasing sea-surface temperatures and ocean heat content
• Melting glacial ice and sea ice
• Rising sea level
• Increasing humidity
In Table 11.1, you will find a summary of the key findings of the IPCC Fifth Assessment Report, which in- cludes a review of the indicators discussed in this sec- tion as well as a preview of climate change causes and forecasts ahead in this chapter. Please refer back to this table as you read.
Temperature
In previous chapters, we discussed the rise in atmospheric temperatures during this century. In Chapter 5, The Human Denominator, Figure HD 5c, presents an important graph of data from four independent surface-temperature records showing a warming trend since 1880. These re- cords, each collected and analyzed using slightly differ- ent techniques, show remarkable agreement. Figure 11.17 plots the NASA temperature data of global mean annual surface air temperature anomalies (as compared to the 1951–1980 temperature-average base line) and 5-year mean temperatures from 1880 through 2012. (Remember from Chapter 5 that temperature anomalies are the variations from the mean temperature during some period of record.)
The temperature data unmistakably show a warm- ing trend. Since 1880, in the Northern Hemisphere, the years with the warmest land-surface temperatures were 2005 and 2010 (a statistical tie). For the Southern Hemi- sphere, 2009 was the warmest in the modern record. In Chapter 5, we saw that the period from 2000 to 2010 was the warmest decade since 1880 (look back to Figure 5.17 on page 135). The data from long-term climate recon- structions of temperature point to the present time as the warmest in the last 120000 years (see Figure 11.11). These reconstructions also suggest that the increase in tempera- ture during the 20th century is extremely likely (within a confidence interval of greater than 95%–100%*) the larg- est to occur in any century over the past 1000 years.
*The Intergovernmental Panel on Climate Change uses the fol- lowing as standard references to indicate levels of confidence in predictions concerning climate change: virtually certain >99% probability of occurrence; extremely likely >95%; very likely >90%; likely >66%; more likely than not >50%; unlikely <33%; very unlikely <10%; and extremely unlikely <5%.