Key Questions about Climate Change

Is the lower atmosphere becoming warmer? The global average surface thermometer record shows warming of 0.4 ° – 0.8° C during the past century. The 2000s were the warmest decade since measurements began in 1850s. http://www.ucsusa.org/global_warming/science_and_impacts/science/global-thermometer-still-climbing.html use these graphs for the following points http://www.climate.gov/#climateWatch Of 144 glaciers monitored since 1900, 142 have retreated, and retreats overall closely mirror global average temperature changes. Global average sea level has risen 15 cm. in past century. Thermal expansion of seas and glacier melting are key contributors. Geological data suggest seas rose at 0.1 to 0.2mm per yr. over last 1000 yrs., but 1 to 2 mm per year in past 100 yrs. (tide gage data)

Is the lower atmosphere becoming warmer? (con’t) Average area of Arctic sea ice cover has decreased by about 2.8% per decade since 1973, when systematic monitoring began. Average Arctic sea ice thickness has declined by 40%, from 3.1 to 1.9 meters. Sub-surface ocean temperatures (upper 300m) have warmed 0.18°C since 1950.

2. Have human activities caused global warming? Some background: Showing human causation requires both demonstrating 1) that human emissions can account for the observed warming trends, and 2) showing that other potential explanations cannot. The past earth climate has shown significant temperature variability long before human societies were capable of causing it.

Natural Causes of Temperature Variability: Earth-sun orbital variations – Average earth-sun distance varies over ~100,000 yr. cycle Seasonal timing of perihelion varies in ~26,000 yr. cycle Axial tilt varies from ~22° to 25° over a ~40,000 yr. cycle There is strong scientific consensus that these cycles are associated with the cycling between ice ages and warm interglacial periods over the past few hundred thousand years. These changes are slow and thus unlikely to have caused the rapid warming of the last 100 yrs. 2. Tectonic activity affects latitudinal locations of continents and geochemical processes that can affect total snow cover and atmospheric chemistry. Such changes are too slow (millions of years) to have caused the rapid warming of past 100 yrs. 3. Volcanic eruptions put dust, droplets and ash into the atmosphere, blocking sunlight and cooling temperatures. No trends in recorded eruption history suggest sustained reductions in past few centuries. On 2, if continents were to move toward the equator, less land would be snow covered and thus the climate would warm. Also, because chemical reactions between rainwater and exposed rock remove CO2 from the atmosphere, changes in worldwide rainfall can change amount of CO2 in the atmosphere.

Natural Causes (con’t) Solar output has showed little or no upward trend since 1978, when satellites began measurement, only periodic variation of less than 0.1% in 11 yr. cycles. Earlier records of sunspot activity (related to solar output) suggest an increase in solar radiation dating back ~two centuries, but not recently. IPCC Scientists believe that at most 20% of warming of past decades can be attributed to increased solar output. Earth system internal variability, such as the El Nino-Southern Oscillation (warming), can have large effects on temperatures. However, between 1000 and 1800, the overall proxy record shows nothing similar to the rate of warming seen since the late 19th century. The 1990s & 2000s appear to be the warmest decades in 1000 yrs. Therefore, there is no compelling evidence that forcing mechanisms (#1-4), or internal variability (#5), or a combination thereof can account for the rate of warming of the past century. 5. The proxy record includes tree ring analysis, ice cores, coral reef chemistry, ocean sediments, boreholes, and more.

If natural causes and variability do not appear to have caused most of recent warming, are human activities sufficient to have caused it? Human activities have increased the concentrations of CO2 (30%) and other greenhouse gasses over past few centuries. Basic physics provide support for the warming role of greenhouse gasses, as do climate models that are based on physics, and empirical observations. The Intergovernmental Panel on Climate Change has concluded that “…most of the observed warming over (the years 1950-2012) is likely to have been due to the increase in greenhouse gas concentrations.” Mid century period of cooling attributed to increased reflection of sunlight from increasing sulfur emissions.

Remember the process in which greenhouse gasses help warm the atmosphere… Some of the shortwave radiation from the sun penetrates the atmosphere, and is absorbed by the earth’s surface. Then, the earth emits longwave radiation, some of which is absorbed by the greenhouse gasses in the troposphere. Thus, solar radiation warms the earth’s surface, and then earth radiation warms the troposphere. (Note: surface air is also warmed through turbulent heat transfer from the heated earth.)

3. What future changes in global climate are expected? Future temperatures are likely to be driven mostly by future concentrations of greenhouse gasses, which will depend on future emissions, which will be driven by population increase world economic growth technological trends policies historical events such as major wars, political transitions, or emergence of epidemic diseases Global warming is likely to continue as greenhouse gas concentrations rise. The higher N. latitudes are expected to continue to show the greatest temperature changes Is the intensity of hurricanes and other storms likely to increase?

4. What are likely impacts of global warming ? Island nations and low income countries will have the most difficulty mitigating negative impacts Food security risks are greatest for those who are poor, isolated, landless, and/or living near sea level (example Bangladesh) 280 million people depending on coral reef environments for a significant part of their food intake will need to migrate or import more food High latitude regions may see increased grain production (mostly wealthy countries including Canada, Scandinavian nations, Russia) Expansion of the latitudes and altitudes of diseases, including malaria, dengue fever, yellow fever (all mosquito vectored). Regions relying on snowmelt waters will have to make changes. Second and third order impacts are difficult to predict, but can be potentially devastating. Example: Western Pine Beetle damage http://www.aibs.org/bioscience-press-releases/resources/June.pdf http://www.nytimes.com/2008/11/18/science/18trees.html?em Warmer temperatures cause trees to be more drought stressed, and thus are less able to secrete enough pitch to immobilize larvae. Also, extreme winter cold used to kill many overwintering beetles, but winters of past 30 years are not so cold.