Download Chapter 10 Chapter 10 – Climate Change

Environmental Transport and Fate
Chapter 10
–
Climate Change
Benoit Cushman-Roisin
Thayer School of Engineering
Dartmouth College
Preamble
Scientists initially talked about “global warming” then switched to “climate change”
and now talk about “climate change and variability.”
Beware of people who still use the expression “global warming”. This is usually a
clue that the person is not a scientist and does not know what he/she is talking
abo t
about.
Also stay away from documentation that features the like of:
1
Recall from the previous lecture that our
terrestrial climate is a balance primarily
between two cooling effects (albedo and
hydrological cycle) and one warming
effect (greenhouse effect).
Water vapor is by far the most important
greenhouse gas. Next to it is carbon
dioxide (CO2), which traps infrared
radiation from the earth’s surface in the
range of 13-to-17 m.
As we all know, combustion of fossil
fuels and tropical deforestation have
combined to increase the CO2 content
of the atmosphere.
Methane (CH4) is another potent
greenhouse gas, with sources
associated with agriculture and landfills.
2
In other words, our current CO2 atmospheric concentration is higher than it has been in
at least the 400,000 years (650,000 years, to be more precise).
… but not since ever.
3
(test.uppergreenside.org/2007/02/)
Emissions by country – in the past
4
… and projections for the future.
The terrestrial carbon cycle
(www.uwsp.edu/.../b
biogeochemical_cycles.html)
Only about half of the anthropogenic CO2 emissions (5.5 GtC/yr) remains in the atmosphere.
The other half is taken up by the ocean (2.5 GtC/yr) and land vegetation (0.2 GtC/yr).
5
What about methane (CH4),
the other potent greenhouse gas?
According to the theory, an increase in CO2 and CH4 should cause an increase in surface
temperature.
Where are we going to get a proof or confirmation?
And, how can we know where we are heading?
There are three avenues.
1. Looking at recent data
(direct temperature measurements by thermometers or satellites)
… but they only give a glimpse at the present and recent past.
2. Paleoclimatic data
(past temperatures inferred from coral, tree rings, ice bubbles, etc.)
… but they cannot shed light on an anthropogenic scenario.
3. Computer models
(inference about what may happen by means of mathematical modeling)
… but these are only as good as their assumptions/parameterizations;
they also disagree with one another.
6
Recent tracking of the global temperature
Increase over the last 33 years = 0.45oC.
Trend = 0.014oC/year.
7
You may have seen the so-called “hockey-stick curve”, which has led many to believe
that we have already experienced a unique temperature rise.
(M
Mann, Bradley & Hughes, 1998)
This plot made its way into the Third Assessment Report of the Intergovernmental
Panel for Climate Change (IPCC).
However…
… this curve has been
under attack.
8
Mann et al. (1998) made some convenient errors.
This is the corrected version.
(S. McIntyre & R. McKitrick, 2003, Energy & Environment, Vol. 14, No. 6, pages 751-771.)
Positive and Negative Feedbacks in the Climate System
POSITIVE FEEDBACKS
1. T ↑, melting of ice & snow, less reflective surface, albedo ↓,
earth absorbing more from sun,
sun T ↑
2. T ↑ ,ocean T ↑, ocean’s ability to take CO2 ↓ , atmospheric CO2 ↑, greenhouse effect
intensified, T ↑
3. T ↑, warmer polar regions, fresher waters at high latitudes, more buoyant waters, less
dense-water formation, Gulf Stream no longer going as far north, heat remains at mid
latitudes, mid-latitude T ↑
4. T ↑, evaporation ↑, some dry regions become humid, H2O greenhouse effect ↑, T ↑
5. T ↑, more vegetation, darker earth, albedo ↓, T ↑
(T stands for temperature, an upward arrow means an increase, and a downward arrow a
decrease.)
9
NEGATIVE FEEDBACKS
1. T ↑, evaporation ↑, intensified hydrological cycle, greater short circuit of greenhouse
effect, radiation from earth surface ↓, T ↓
2. T ↑, evaporation ↑, lower cumulus clouds and less moisture at high altitudes, albedo ↑,
heat taken from sun ↓, T ↓ (so-called Iris Effect)
3. T ↑, more vegetation, take-up of CO2 ↑, atmospheric CO2 ↓, greenhouse effect ↓, T ↓
4. T ↑ at high-latitudes, melting of glaciers, fresher waters in high-latitude ocean, less
deep-water formation, Gulf Stream no longer reaching as far north, T ↓ at high-latitudes
(T stands for temperature
temperature, an upward arrow means an increase
increase, and a downward arrow a
decrease.)
The general oceanic circulation, called the conveyor belt, can play the role of both
positive and negative feedback:
(www.srh.noaa.g
gov/jetstream/ocean/circulation.htm)
POSITIVE FEEDBACK T ↑, warmer polar regions, fresher waters at high latitudes,
less dense-water formation, Gulf Stream no longer going as far north, heat remains at
mid latitudes, T ↑ at mid-latitudes.
NEGATIVE FEEDBACK T ↑ at high-latitudes, melting of glaciers, fresher waters in
high-latitude ocean, less deep-water formation, Gulf Stream no longer reaching as far
north, T ↓ at high-latitudes.
10
Of course, some feedbacks are
more important than others.
(TOA = Top of Atmosphere = Tropopause)
A number of people, most often non scientists, have
agendas and therefore aim to make a certain point.
With that perspective, they frequently cherry pick the
evidence and mention only the feedbacks that
strengthen their point.
Beware. Many exaggerated claims have been voiced.
To avoid the trap, let’s see what the professionals are
saying.
11
Just a few of Al Gore’s errors
Polar bears
Not endangered, population has been increasing (+300% since 1950).
“Significant numbers have died as a result of global warming” came from a single
scientific paper relating the death of 6 polar bears as a result of a severe storm, which
could not be linked with climate change.
Hundreds of polar bears are killed each year by hunting and in self-defense.
Melting snow on Mt. Kilimanjaro
Al Gore claims that global warming and deforestation is causing the melting of the
snowcap on Mt. Kilimanjaro.
The melting and freezing of moisture on top of Kilimanjaro appears to be part of “a
natural process of dry and wet periods.” (Jaap Sinninghe Damste , U. Utrecht) The
present melting is not the result of “environmental damage caused by man.” The current
icecap was not the result of a colder atmosphere but of large amounts of precipitation
which fell at the beginning of the Holocene period, about 11,000 years ago and have
been slowly melting/evaporating ever since.
since
Sea-level rise
Al Gore states that a sea-level rise of up to 20 ft would be caused by melting of either
west Antarctica or Greenland in the near future.
Data show that ice on Antarctica is not significantly decreasing overall. Melting of all of
Greenland’s ice could occur over many centuries and create a sea level rise of 3.2 ft
over the next millennium (IPCC finding).
Intergovernmental Panel for Climate Change (IPCC)
Fourth Assessment: Climate Change 2007 – The Physical Science Basis
(Fifth Assessment scheduled to be released in 2014)
Scientific summary divided into four parts:
1.
2.
3.
4.
Global concentrations of CO2 and other greenhouse gases
Radiative forcings of atmospheric impurities
Observed changes in climate
Attributing climate change
Nomenclature:
Virtually
Vi
t ll certain:
t i > 99% probability
b bilit off occurrence
Extremely likely: > 95% probability of occurrence
Very likely: > 90% probability of occurrence
Likely: > 66% probability of occurrence
More likely than not: > 50% probability of occurrence
Unlikely: < 33% probability of occurrence
Very unlikely: < 10% probability of occurrence
Extremely unlikely: < 5% probability of occurrence
Side note: If 10 “very likely”
claims are made in the report,
then the chance that all are
correct is (0.9)10 = 0.35, and
there is a 65% chance that at
least one is wrong.
12
First off, most people have lifted the word “unequivocal” from the following sentence:
“Warming of the climate system is now unequivocal, as is now evident from
observations of increased in global average air and ocean temperatures,
widespread melting of snow and ice, and rise global average sea level.”
(IPCC – Summary For Policymakers – February 2007, page 5)
Note that the word is NOT used in relation to increases in greenhouse gases, leaving
the possibility that there is some natural component.
One has to go elsewhere in the report to find the connection:
“Most of the observed increase in globally averaged temperatures since the
mid-20th century is very likely due to the observed increase in anthropogenic
greenhouse gas concentrations.”
(IPCC – Summary For Policymakers – February 2007, page 10)
Another misleading statement often made:
CO2 emissions
Graph used in Al Gore’s movie
“An Inconvenient Truth”
13
And, what else does the IPCC say?
1. Concentrations of greenhouse gases
Atmospheric CO2 concentration has increased from about 280 ppm in pre-industrial times
to 379 ppm in 2005
2005, exceeding concentrations in the past 650
650,000
000 years
years.
Rise in CO2 concentration is accelerating, now at 1.9 ppm/year, largest known rate.
Sources of new CO2 are primarily fossil fuel and secondarily changes in land use.
Atmospheric concentration of CH4 has increased from about 715 ppb in pre-industrial
times to 1774 ppb in 2005, exceeding concentrations of the past 650,000 years.
It is very
y likelyy that the rise in CH4 concentration has been caused by
y human activities.
Atmospheric concentration of N2O has increased from about 270 ppb in pre-industrial
times to 319 ppb in 2005.
More than a third of the new N2O is attributable to human activities, primarily agriculture.
2. Radiative forcings of atmospheric impurities:
The combined radiative forcing of CO2, CH4 and N2O combined is estimated at
+2.30 W/m2 (warming).
Radiative forcing of aerosols (sulfate, organic carbon, black carbon, nitrate, dust) is
estimated at –0
0.5
5 W/m2 (cooling).
(cooling)
Radiative forcing of tropospheric ozone (O3) is estimated at +0.35 W/m2 (warming) and of
halocarbons at +0.34 W/m2 (warming).
Changes in surface albedo is estimated at –0.2 W/m2 due to changes in land surface and
+0.1 W/m2 due to deposition of black carbon on snow.
Sum of the preceding numbers = +2.39 W/m2. But when all is said and done (se next
slides), the final number is +1.6 W/m2.
To keep these numbers in perspective, recall that the incident solar radiation is 343 W/m2.
So, we are talking about a 0.5% perturbation to the climate equilibrium.
Rule of thumb (from James Hansen, NASA): 0.75oC rise for every 1 W/m2 of increase in
hear flux → +1.2oC = +2.2oF temperature rise, globally.
14
3. Observed changes in climate:
Increase in solar irradiance sine 1750 is causing a radiative forcing of +0.12 W/m2
(much less than previously estimated).
11 out of the 12-year period 1995–2006 rank among the 12 warmest years since 1850.
Global temperature increase is 0.074oC/decade averaged over the last 100-year
period from 1906 to 2005.
Global temperature increase is 0.13oC/decade over the last 50 years.
Urban heat island effect is real but local and negligible (less than 0.006oC/decade,
on land only, zero at sea).
The average atmospheric water vapor has increased since the 1980s
1980s. The increase is
broadly consistent with the extra water vapor that warmer air can hold.
Observations since 1961 show an average temperature increase in the ocean down to
depths of at least 3000 m. The ocean has been absorbing more than 80% of the heat
added to the climate system.
Warming of the ocean causes water to expand, contributing to sea level rise.
15
Rate of sea-level rise (in m per century)
Cause of sea-level rise
1961 – 2003
1993 – 2003
Thermal expansion of water
0.042 ± 0.012
0.16 ± 0.05
Melting glaciers and ice caps
0.050 ± 0.018
0.077 ± 0.022
Melting of Greenland ice sheets
0.05 ± 0.12
0.21 ± 0.07
Melting of Antarctic ice sheets
0.14 ± 0.41
0.21 ± 0.35
Estimated sea-level rise
0.11 ± 0.05
0.28 ± 0.07
Ob
Observed
d sea-level
l
l rise
i
0 18 ± 0.05
0.18
0 05
0 31 ± 0.07
0.31
0 07
Difference: Observed – Estimated
0.07 ± 0.07
0.03 ± 0.10
16
According to Princeton climate model, based on business-as-usual scenario.
3. Observed changes in climate (cont’d):
Arctic temperatures have increased at almost twice the global rate in the past 100 years.
They exhibit strong decadal variability. A warm period was observed from 1925 to 1945.
Satellite data show that the extent of Arctic sea ice has shrunk by 2.7% per decade since
1978. (This means that 21% of summer ice has been lost since 1978.)
Precipitation has increased in eastern parts of North and South America, northern Europe,
and northern and central Asia.
Drying has been observed in the Sahel, the Mediterranean, southern Africa, and parts of
southern Asia. Caution: Highly variable, limited data.
Precipitation changes over the ocean can be inferred from changes in salinity:
freshening of mid and high latitudes,
latitudes increased salinity in tropics
tropics.
Cold days and cold nights have been less frequent, while hot days and hot nights have
become more frequent.
Mid-latitude westerly winds have strengthened in both hemispheres since the 1960s.
17
4. Attributing climate change:
Very likely that most observed increased in globally averaged temperatures since the
mid-20th century is due to increase in anthropogenic greenhouse gas concentrations
(up from likely as stated in IPCC’s Third Assessment report).
Likely that increases in greenhouse gas concentrations alone would have caused more
warming, because volcanic eruptions and aerosols have offset some of the warming.
Extremely likely that global change in the past 50 years can be explained without
external forcing (such as increased in solar radiation)
Very likely that it is not due to natural causes alone.
Likely that there has been significant anthropogenic warming over the past 50 years
over each continent, except Antarctica.
Difficulties and uncertainties remain in simulating and attributing observed temperature
changes at smaller scales.
Likely that anthropogenic forcing has contributed to changes in wind patterns.
Likely that temperatures of most extreme hot nights, cold nights and cold days have
increased to due anthropogenic forcing.
18