Key Questions about Climate Change
Is the lower atmosphere becoming warmer?
1) Global temperature rose at an average rate of about 0.13°C per decade for the past 50 years. 2015, 2016, 2017, 2018, 2019 are the warmest since thermometer measurements began in 1850s. http://www.climate.gov/
2) Globally representative mountain glaciers with the
longest, most reliable records lost 72 feet of thickness 1980 – 2016. (57 feet by 2014)
3) Global average sea level rise has accelerated from 1.7 mm/year throughout most of the twentieth century to 3.2 mm/year since 1993. (thermal expansion & melting) 4) Sub surface ocean temperatures (upper 75m) have
warmed 0.09 to 0.13°C per decade since 1971 (when most thorough measurements began)
Source: http://www.climate.gov/
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:
1. Earth-sun orbital variations –
a) Average earth-sun distance varies over ~100,000 yr. cycle
b) Seasonal timing of perihelion varies in ~26,000 yr. cycle (currently in cold phase for N. Hemisphere: July 4, warm Jan 3)
c) Axial tilt varies from ~22° to 25° over a ~40,000 yr. cycle (currently in decreasing phase, 23.44 degrees)
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.
Natural Causes (con’t)
4. 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.
5. Earth system internal variability, such as the episodic 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.
6. 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.
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.”
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, which warms up. 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 & coastal populations, 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 https://www.youtube.com/watch?
v=NdZhD8zhiS0)
• Hundreds of millions of 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 killing millions of forest acres http://www.wou.edu/~mcgladm/Geography%20313%20Pacific%20Northwest/for
%20exam%201%20physical%20landscape/optional/optional%20bark%20beetle
%20outbreaks%20related%20to%20warming.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. Extreme winter cold used to kill many overwintering beetles, but winters of past 30 yrs. are not so cold. Longer warm seasons allow more reproduction.
