Ch 19 Climate Change Introduction to Climate Change Throughout time, the earth's climate has always been changing produced ice ages Hence, climate variations have been noted in the past what physical processes create natural fluctuations in the earth's climate? Is there an anthroprogenic influence on climate change? is so, what will its effect be??? How do we measure climate change in the past? How are we predicting climate change in the future? 1
Determining Past Climate Change Ocean Floor Sediments the sediment contains calcium carbonate shells from organisms that have lived near the earth's surface in the past the type of calcium carbonate shell can tell you something about temperature since some live only within narrow temperature ranges. Oxygen Isotopes O 16 will evaporate more readily than O 18 since it is lighter Hence, during a warm period, the relative amount of O1 8 will increase in the ocean waters since more of the O 16 is evaporating Hence, looking at the ratio of O 16 to O 18 in the past can give clues about global temperatures. Ice cores from glaciers can also give you similar information Dendrochronology one ring per year distance between tree rings can tell you something about temperature and moisture fluctuations related to climatic variability Altering the microclimate. Notice in the picture that the leaves are still on the tree near the streetlight. Apparently, this sodium vapor lamp emits enough warmth and light during the night to trick the leaves into behaving as if it were September rather than the middle of November. 2
Extent of glaciation about 18,000 years ago over (a) North America and over (b) western Europe. If all the ice locked up in glaciers and ice sheets were to melt, estimates are that this coastal area of south Florida would be under 65 m (213 ft) of water. (a) Sea surface isotherms ( C) during August 18,000 years ago and (b) during August today. 3
Climate through the Ages - Long-Term Temperature Record Throughout much of earth's history - global climate was 8 C-15 C warmer than today s climate polar regions ice free Warm climate was periodically interrupted by periods of glaciation 700 million years ago (mya) 300 mya more recently, the Pleistocene epoch, or simply the Ice Age occurred 2 mya More recently, North American glaciers reached maximum thickness 18,000-22,000 years ago --> Land connecting Asia and N. America along the Bering Sea was exposed since a great deal of the water was locked up in glaciers 14,000 y.a.,, glaciers started to retreat as temps rose 11,000 y.a., temps suddenly fell - referred to as Younger- Dryas Younger-Dryas lasted about 1000 years, then avg temps increased, glaciers retreated. 6,000-5,000 y.a., climate was about 1 C warmer than normal Climate through the Ages - The last 1000 Years Notice that for most of the last 1000 years, the climate has been cooler than normal. The period from about 1550-1850 is referred to as the Little Ice Age. 4
Climate through the Ages - Recent Trends Since late 1800's, we've been in a warming trend Avg. temp has increased by about 1 C The eight hottest years of this century have occurred since 1979 Corrected data suggest a warming trend of 0.3-0.6 C over the last 100 years. So, is the recent warming due to anthroprogenic causes or is it due to natural fluctuations in climate? enhanced greenhouse effect due to increasing CO2 concentrations in the atmosphere This question is still a subject of debate, but has very important consequences.. Climate Change and Feedback Mechanisms Feedback mechanism - when physical processes in the earth-atmosphere system further impact the initial change if the impact is such that the initial perturbation is enhanced, then it is called a positive feedback mechanism --> water vapor-greenhouse feedback if the impact is such that the initial perturbation is reduced, or weakened, then it is called a negative feedback mechanism Earth albedo feedback 5
Climate Change - Plate Tectonics Theory of Plate Tectonics - Continental Drift Earths outer shell is composed of plates --> they move at a rate of about 3 cm per year Affect of more land at higher latitudes: alter ocean currents and therefore heat transport alter global atmospheric circulation more glaciers over land, higher albedo, cooler temps. Plate movement also generates more volcanic activity hence, when the plates are on the move, have more volcanic eruptions -> emit more CO2 into atmosphere global temps to rise. if there is little movement, volcanic activity decreases -> so CO2 concentrations are lower in the atmosphere -> avg temp decreases About 180 million years ago Today The earth is composed of a series of moving plates. The rate at which plates move (spread) may influence global climate. During times of rapid spreading, increased volcanic activity may promote global warming by enriching the CO 2 content of the atmosphere. 6
Climate Change - Milankovitch Theory - Eccentricity Cycle Climate change due to variations in the earth's orbit - Milankovitch Theory 1) eccentricity cycle - the earth's orbit around the sun is elliptical. the shape of the ellipse (eccentricity) varies from less elliptical to more elliptical back to less elliptical and take about 100,000 years to complete this cycle. currently, we are in an orbit of low eccentricity (near circular). Data analysis for the last 800,000 years of deep-ocean sediments show that ice coverage is a maxima every 100,000 years. This matches the Eccentricity cycle period For the earth s orbit to stretch from nearly a circle (solid line) to an elliptical orbit (dashed line) and back again takes nearly 100,000 years. Climate Change - Milankovitch Theory- Precession Cycle 2) Precession cycle - The earth is wobbling about it's axis of rotation like a spinning top To make one complete cycle takes about 23,000 years in 11,000 years, the seasons will switch times during year why? (a) Like a spinning top, the earth s axis of rotation slowly moves and traces out the path of a cone in space. (b) Presently the earth is closer to the sun in January, when the Northern Hemisphere experiences winter. (c) In about 11,000 years, due to precession, the earth will be closer to the sun in July, when the Northern Hemisphere experiences summer. 7
Climate Change - Milankovitch Theory- Tilt Cycle 3) Tilt Cycle - currently, the axis of rotation for the earth is tilted at 23.5 However, this value changes from a minimum of 22.5 to a maximum of 24.5 and takes 41,000 years to complete one cycle The Milankovitch cycles and plate tectonics are not the only natural factors which can affect global climate, there are other factors to consider: amount of dust and aerosols in the atmosphere reflectivity of ice sheets concentrations of trace gases amount of clouds Climate Change - Aerosols in the troposphere Aerosols are tiny liquid and solid particles They enter the troposphere by: factory and auto emissions agricultural burning / wild fires ocean - phytoplankton produce dimethylsulphide (DMS) - DMS forms SO2 in atmosphere which in turn produces sulfate aerosols Aerosol concentrations are increasing with time. Tropospheric aerosol effect on climate: reflects incoming solar radiation - cooling affect absorbs LW radiation - warming effect (especially the black sooty aerosols emitted through fossil fuel and biomass burning) The net effect of tropospheric aerosols is thought to be one of cooling. 8
Climate Change - Aerosols in the Stratosphere Largely injected by volcanic eruptions Remember, the stratosphere is a stable layer, so lighter, smaller aerosols will have long residence times Aerosols reflect and absorb short wave radiation As a result aerosols produce warming in the stratosphere and cooling in the troposphere Recent significant eruptions: El Chichon - April 1982 Mount Pinatubo - June, 1991 Climate Change - Mt. Pinatubo Mt. Pinatubo injected 20 million tons of sulfur dioxide into the stratosphere! The sulfur dioxide was observed around the globe in the equatorial regions average hemispheric temperatures dropped by 0.2-0.5 C for a period of 1-3 years. Sulfur dioxide plume (dark red and green areas) from the eruption of Mount Pinatubo Global air temperature before and after Mt. Pinatubo 9
Climate Change - Variation in Sun Spot Activity Huge magnetic storms that show up as dark (cool) areas on the suns surface When there are more sunspots, the sun is emitting more energy, hence, the amount of energy incident on the earth increases so, one could imagine warmer climate during a sun spot maximum and cooler climate during a minimum. sun spot period is about 11 years Climate Change - Increasing Concentrations of CO2 and other Green House Gases CO 2 natural and anthropogenic sources recent increase due to fossil-fuel combustion and deforestation CH 4 natural and anthropogenic sources about 1/2 of current emissions are anthropogenic (land fills, natural gas, agriculture) N 2 O natural and anthropogenic sources nitrogen-based fertilizers Other important Greenhouse Gases: CFCs, ozone, water vapor 10
Projected surface air temperature changes from different climate models. Climate Change - CO 2 Feedback mechanisms Other trace gases like N2O, CH4, and CFCs are also increasing - these are also greenhouse gases increased temps will enhance evaporation from oceans -> increased water vapor in atmosphere -> enhanced greenhouse effect increased temps will enhance evaporation -> increase amount of low clouds -> increase earth's albedo CO 2 will dissolve into the oceans Vegetation will remove CO 2 and grow more vigorously 11
Global average projected temperature changes ( C) from 1990 to 2100 using climate models with six different scenarios. Each scenario describes how the average temperature will change based on different concentrations of greenhouse gases and various forcing agents. Climate Change - Possible Consequences of Global Warming Temperature: Globally averaged surface temperature is projected to increase by 1.4 to 5.8 ºC over the period 1990-2100. Greater relative warming in the higher latitudes Land areas will warm more and faster than ocean areas The projected rate of warming is much larger than the observed changes during the 20th century and is very likely to be without precedent during at least the last 10,000 years, based on palaeoclimate data. Projected changes in surface air temperature due to a doubling of CO2 and human-induced sulfide emissions with an Atmospheric Ocean General Circulation Model (AOGCM). 12
Precipitation: Globally averaged water vapor, evaporation and precipitation are projected to increase. NOTE: At the regional scale, increases and decreases will be observed. Extreme Weather: More hot days and heat waves are likely over nearly all land areas Frost days and cold waves are very likely to become fewer Frequency of extreme precipitation events is projected to increase General drying (increased drought frequency) of all mid-continental areas during summer. Glaciers and Snow Cover: Glaciers and ice caps will continue their widespread retreat during the 21st century and NH snow cover and sea ice are projected to decrease further. NOTE: The Antarctic ice sheet is likely to gain mass because of greater precipitation Sea Level Rise: A sea level rise of 0.09 0.88 meters is projected for 1990-2100. Thermal expansion of the oceans Loss of mass from glaciers and ice caps The Source of Global Warming Q: is the observed warming over the last 50-100 years due to natural climate variability, human influence, or both?. natural forcing alone is unlikely to explain the recent observed global warming or the observed changes in vertical temperatures structure of the atmosphere. In light of new evidence and taking into account the remaining uncertainties, most of the observed warming over the last 50 years is likely to have been due to the increase in greenhouse gas concentrations. Figure and quotes from Technical Summary from IPCC Working Group I 13