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Key Questions about Climate Change
1. 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
1990s were the warmest decade since measurements
began in 1850s.
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)
1. Is the lower atmosphere becoming
warmer? (con’t)
4. 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.
5. 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
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.
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.
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)
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 NinoSouthern 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.
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
The Intergovernmental Panel on Climate Change has
concluded that “…most of the observed warming over
(the years 1950-2000) 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. 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
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 latitudes are expected to continue to show
the greatest temperature changes
• Is the intensity of hurricanes is 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
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.