Lecture 35: The Global Warming Debate Ch. 18

The Global Warming Debate Ch. 17, Ch Is global warming real? (Or is global warming happening?) 2.What is the evidence? 3.Is global warming part of natural cycles? Or is it caused by humans? Why? 4.Why do we care about global warming? 5.How much warming is caused by humans?

Data over the globe (land and sea). Warming periods: (by 0.5°C), the mid-1970s to present. The warmest decade: the 1990s. The warmest year: 2005, The warmest decade: the 1990s. The warmest year: 2005, sixth warmest year on record. Over last 25 years warming ~ 0.5 C Over past century warming ~ 0.75 C Cooling periods: Temperature Trends in the 20 th Century

Surface Air Temperature Trends Over the Past Century Warming greatest at night over northern mid-to-high latitude land Stronger warming during winter and spring Greater than the global average in some areas Cooled in some areas (southern Mississippi Valley in USA)

USA 100-yr Temperature Trends Temperatures have actually cooled, e.g., over the southern Mississippi Valley in North America.

The Earth’s Climate History 1.Over the last century, the earth’s surface temperature has increased by about 0.75°C (about 1.35°F). 2.Little Ice Age = Cooling during 1,400 A.D. – 1,900 A.D. (N.H. temperature was lower by 0.5°C, alpine glaciers increased; few sunspots, low solar output) 3.Medieval Climate Optimum (Warm Period) = Warming during 1,000 A.D. – 1,300 A.D. in Europe and the high-latitudes of North Atlantic (N.H. warm and dry, Nordic people or Vikings colonized Iceland & Greenland) 4.Holocene Maximum = 5,000-6,000 ybp (1°C warmer than now, warmest of the current interglacial period) 5.Younger-Dryas Event = 12,000 ybp (sudden drop in temperature and portions of N.H. reverted back to glacial conditions) 6.Last Glacial Maximum = 21,000 ybp (maximum North American continental glaciers, lower sea level exposed Bering land bridge allowing human migration from Asia to North America) 7.We are presently living in a long-term Icehouse climate period, which is comprised of shorter-term glacial (e.g., 21,000 ybp) and interglacial (e.g., today) periods. There were four periods of Icehouse prior to the current one. 8.For most of the earth’s history, the climate was much warmer than today.

Uncertainties about the Global Warming of the Past Century Skeptics a. No warming discernable in data b. If warming, not from human activities The sunspot cycleThe role of ozone The role of cloudsThe role of sulphate aerosols The role of dustThe role of oceans c. If warming, and from human activities, maybe warming not so badwarming not so bad d. If warming, and from human activities, and it is a problem, then engineer a solution to pull CO 2 out of the atmosphere, e.g., through carbon sequestration, while conducting other business as usual (i.e., don’t cut CO 2 emissions). Uncertainties in the temperature data early records are subject to instrumental error and improper sitting uneven land and sea stations too many Northern Hemisphere records urban heat island effect mismatch between satellite, balloon and surface data ALL THESE HAVE BEEN CAREFULLY ACCOUNTED FOR OR MOSTLY RESOLVED

Natural Warming and Greenhouse Effects

Observed Temperature Changes and Natural Contributions Observed trend 0.6°C/century

Components of 2 × CO 2 Warming

ModelModel Simulations of 2 x CO Sensitivity Model Simulations of 2 x CO 2 Sensitivity Model

Earth’s CO2 Sensitivity from Climate Models Climate ModelsClimate Models

Earth’s CO2 Sensitivity from Earth History

Uncertainties about the Global Warming of the Past Century Skeptics a. No warming discernable in data b. If warming, not from human activities The sunspot cycleThe role of ozone The role of cloudsThe role of sulphate aerosols The role of dustThe role of oceans c. If warming, and from human activities, maybe warming not so badwarming not so bad d. If warming, and from human activities, and it is a problem, then engineer a solution to pull CO 2 out of the atmosphere, e.g., through carbon sequestration, while conducting other business as usual (i.e., don’t cut CO 2 emissions). Uncertainties in the temperature data early records are subject to instrumental error and improper sitting uneven land and sea stations too many Northern Hemisphere records urban heat island effect mismatch between satellite, balloon and surface data ALL THESE HAVE BEEN CAREFULLY ACCOUNTED FOR OR MOSTLY RESOLVED

Radiative Effects of Greenhouse Gases

Effects of Increases in Greenhouse Gases on Radiation

Marine Aerosols from Algal Emissions can affect Climate O’Dowd et al., Marine aerosol formation from biogenic iodine emissions, Nature 417, (2002)

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