1. Climate Drivers Greenhouse Gases
Complexity, climate change and human systems
HCOL 185

2. External and Internal Drivers of Climate External Drivers
-Sunspot Cycles
-Orbital Variations
Internal Drivers
-Plate Tectonics
-Volcanic Activity
-Albedo
-Greenhouse Effect

3. Climate Drivers
Greenhouse gases

4. Solar and heat energy passing through the climate system is reflected, absorbed and re-radiated in many different ways
Slide showing energy flux
through atmosphere
The intensity of short-wave sunlight reaching the earth’s outer atmosphere is very steady, ranging by only about a tenth of a percent from its mean value of about 1370 watts per square meter during the sunspot cycles. Although this value is called the solar constant, there is evidence that it can vary by larger amounts (a few tenths of a percent) on century and millennial time scales.
When averaged around the entire planet, this incoming energy is about 342 watts of energy for each square meter of the earth’s surface.
About 22% of this radiation (77 W/m2) is reflected back to space by clouds, atmospheric dust and other aerosols in the atmosphere, and another 9% by the earth’s surface. The fraction of incoming solar energy reflected to space through these processes is known as the planetary albedo.
Changes in the amount of dust in the atmosphere, the extent and type of clouds and the reflective properties of the earth surface can cause the average planetary albedo to vary considerably by season, from year to year and over longer time scales.
The remaining 69% of the sun’s energy absorbed by the atmosphere and the earth’s surface heats the climate system, drives the hydrological cycle and causes motion in the atmosphere and the world’s oceans.
Heat is transported upward from the earth’s surface and lower atmosphere by latent heat within water vapour, direct thermal convection of warm air upward and the emission of long wave radiative energy towards space.
However, the absorption of outgoing long wave radiation by clouds, aerosols and traces of greenhouse gases within the atmosphere, together with the release of heat from condensation of water vapour at higher levels of the atmosphere, help to retain much of this heat within the troposphere. This retained energy is re-radiated in all directions, much of it back towards the surface. Furthermore, much of the portion of energy re-radiated upwards is again absorbed by other greenhouse gas molecules and aerosols at higher elevations, and the process is repeated.
For a balanced climate, the amount of energy that finally escapes into space is, on average, equal to the absorbed solar energy. However, enough of the outgoing infrared is recycled to increase surface temperatures significantly.
Whenever changes in the intensity of incoming solar radiation or in the atmospheric properties that contribute to the above processes cause an imbalance in the net incoming-outgoing energy flux at the top of the atmosphere, climate change results. Such changes are referred to as ‘radiative forcing’.
Reference: Adapted from IPCC Climate Change 2001: The Scientific Basis Contribution of Working Group I to the Third Assessment Report of the Intergovernmental Panel on Climate Change [Houghton,J.T. et al. (eds.)]. Cambridge University Press, Cambridge, UK., Fig 1.2

5. Estimating the Magnitude of the Natural Greenhouse Effect
Estimating the Magnitude
of the Natural Greenhouse Effect
Net Incoming Solar Energy
Outgoing Heat Energy =
(S0 (1-A) R2)
(4R2kTe4)
where
S0 is the solar constant
A is average albedo, or reflectivity
R is the radius of the earth
k is Boltzmann’s constant
Te is earth’s apparent temperature (seen from space)
In the above equation, the climate is assumed to be in balance, with net loss of infrared energy released from the earth’s surface and atmosphere to space equal (on average) to the net solar energy absorbed by the earth’s climate system.
All parameters in the equation are known except the effective radiating temperature, which is the temperature the earth would appear to have if measured with a radiometer located on the moon.
S0 is w/m2; A is 0.31; R is 6371 km; k is 1.38 x10-16 erg/deg.
The 33ºC difference in average surface temperatures between an earth without an atmosphere and our current climate is the difference between a livable planet and a snowball earth.
Reference: IPCC Climate Change 2001: The Scientific Basis Contribution of Working Group I to the Third Assessment Report of the Intergovernmental Panel on Climate Change [Houghton,J.T. et al. (eds.)]. Cambridge University Press, Cambridge, UK. pp 89-90 Te
equals -19C 
However, average global surface T is + 14C
Natural greenhouse effect warms the
surface by 33C 

6. Primary Contributors to the Natural Greenhouse Effect
~10%
~25%
~65%
Model studies suggest that, if water vapour were the only greenhouse gas present in the atmosphere, its magnitude would be only about 60-70% of that for all greenhouse gases.
If carbon dioxide were the only greenhouse gas present, the effect would be about 25% of that of all gases.
Because of overlapping absorption bands, these numbers are not strictly additive, but provide a broad estimate of relative significance.
Reference: IPCC Climate Change: The IPCC Scientific Assessment [Houghton,J.T. et al. (eds.)]. Cambridge University Press, Cambridge, UK. pg 48.
Why is water vapor not given more attention as a GHG?

7. Absorption Spectra

8. What are the GHG’s? Greenhouse gases include CO2 (Carbon dioxide)
CH4 (Methane)
N2O (Nitrous oxide)
Halocarbons

9. Composition of Dry Air Nitrogen 78.1% Oxygen 20.9% Argon 0.9%
Carbon dioxide %
Neon %
Helium %
Methane %
Nitrogen and oxygen currently make up 99% of the volume of clean (no aerosols) and dry (no moisture) air. These two gasses are very important to life, especially oxygen, but their significance to our weather and climate are minor. Figure 1-10 Lutgens and Tarbuck 2001.

10. The atmospheric lifetime, and Global Warming Potential of GHGs vary considerably
(years)
GWP
(100 yr)
CO2
50-200 1
Methane 12 23
N2O
114
296
CFCs, HCFCs
2-1700
120-14,000
HFCs
12-12,000
PFCs, SF6
,000
,200
In the table, GWP represents Global Warming Potential, a measure of potency (by weight) of the substance as a greenhouse gas relative to that for carbon dioxide.
While non-CO2 greenhouse gases have much lower concentrations and emission rates that that for CO2, they are also much more potent, molecule for molecule and gram for gram.
Some, like SF6, have very long lifetimes in the atmosphere and are so potent that one gram of emissions can enhance the natural greenhouse effect by the same amount as more than 22 kg of CO2.
However, because of the volume of emissions, human emissions of CO2 remain the most important cause for human induced climate change, followed by those for methane, halocarbons, and nitrous oxide.
Source: Adapted from IPCC Climate Change 2001: The Scientific Basis Contribution of Working Group I to the Third Assessment Report of the Intergovernmental Panel on Climate Change [Houghton,J.T. et al. (eds.)]. Cambridge University Press, Cambridge, UK. pp 244,

11. Trends in key human forcings during the past century are dominated by greenhouse gases but partially offset by other factors
Radiative forcing (W/m2)
The effects of rising concentrations of greenhouse gases on radiative forcing has been steadily increasing throughout the past century.
The effects of sulphate and biomass aerosols have meanwhile been towards cooling. These effects are considered to be ‘masking’ some of the greenhouse gas influences, rather than ‘offsetting’, since the aerosol effects would disappear quickly if emissions stopped while that for greenhouse gases with long atmospheric lifetimes persists well after emissions stop.
The cooling effects for stratospheric ozone depletion are only shown for the past two decades, since no reliable data exists prior to this time.
Sources: Adapted from IPCC Climate Change 2001: The Scientific Basis Contribution of Working Group I to the Third Assessment Report of the Intergovernmental Panel on Climate Change [Houghton,J.T. et al. (eds.)]. Cambridge University Press, Cambridge, UK. Figure 6.8 (p 401); Hansen et al in JGR 102:
Year

12. Billion tonnes of carbon per year
Carbon dioxide emissions from human sources have increased steadily over the past century
Fossil fuel
emissions
Billion tonnes of carbon per year
(GtC/yr)
Note that 1 GtC is equal to about 3.7 GtCO2.
Source: CDIAC
Land use change
emissions
Year

13. Figure 2.20
Figure (A) Global mean RFs from the agents and mechanisms discussed in this chapter, grouped by agent type. Anthropogenic RFs and the natural direct solar RF are shown. The plotted RF values correspond to the bold values in Table Columns indicate other characteristics of the RF; efficacies are not used to modify the RFs shown. Time scales represent the length of time that a given RF term would persist in the atmosphere after the associated emissions and changes ceased. No CO2 time scale is given, as its removal from the atmosphere involves a range of processes that can span long time scales, and thus cannot be expressed accurately with a narrow range of lifetime values. The scientific understanding shown for each term is described in Table (B) Probability distribution functions (PDFs) from combining anthropogenic RFs in (A). Three cases are shown: the total of all anthropogenic RF terms (block filled red curve; see also Table 2.12); LLGHGs and ozone RFs only (dashed red curve); and aerosol direct and cloud albedo RFs only (dashed blue curve). Surface albedo, contrails and stratospheric water vapour RFs are included in the total curve but not in the others. For all of the contributing forcing agents, the uncertainty is assumed to be represented by a normal distribution (and 90% confidence intervals) with the following exceptions: contrails, for which a lognormal distribution is assumed to account for the fact that the uncertainty is quoted as a factor of three; and tropospheric ozone, the direct aerosol RF (sulphate, fossil fuel organic and black carbon, biomass burning aerosols) and the cloud albedo RF, for which discrete values based on Figure 2.9, Table 2.6 and Table 2.7 are randomly sampled. Additional normal distributions are included in the direct aerosol effect for nitrate and mineral dust, as these are not explicitly accounted for in Table 2.6. A one-million point Monte Carlo simulation was performed to derive the PDFs (Boucher and Haywood, 2001). Natural RFs (solar and volcanic) are not included in these three PDFs. Climate efficacies are not accounted for in forming the PDFs.

14. Figure 2.22
Figure Integrated RF of year 2000 emissions over two time horizons (20 and 100 years). The figure gives an indication of the future climate impact of current emissions. The values for aerosols and aerosol precursors are essentially equal for the two time horizons. It should be noted that the RFs of short-lived gases and aerosol depend critically on both when and where they are emitted; the values given in the figure apply only to total global annual emissions. For organic carbon and BC, both fossil fuel (FF) and biomass burning emissions are included. The uncertainty estimates are based on the uncertainties in emission sources, lifetime and radiative efficiency estimates.

15. Carbon dioxide (CO2) Sources: Natural: biomass respiration and burning
Human-caused: fossil fuels, cement manufacturing, land-use change (deforestation)
Sinks: plants, oceans, soils, sediments, rocks
Recent increase in CO2 is unprecedented for the last 420,000 years
31% increase since 1750
Source: IPCC TAR 2001

16. 2. Over the last 400,000 years the Earth's climate has been unstable, with very significant temperature changes, going from a warm climate to an ice age in as rapidly as a few decades. These rapid changes suggest that climate may be quite sensitive to internal or external climate forcings and feedbacks. As can be seen from the blue curve, temperatures have been less variable during the last years. Based on the incomplete evidence available, it is unlikely that global mean temperatures have varied by more than 1°C in a century during this period. The information presented on this graph indicates a strong correlation between carbon dioxide content in the atmosphere and temperature. A possible scenario: anthropogenic emissions of GHGs could bring the climate to a state where it reverts to the highly unstable climate of the pre-ice age period. Rather than a linear evolution, the climate follows a non-linear path with sudden and dramatic surprises when GHG levels reach an as-yet unknown trigger point.
Period of Record
414,085-2,342 years BP
Methods
In January 1998, the collaborative ice-drilling project between Russia, the United States, and France at the Russian Vostok station in East Antarctica yielded the deepest ice core ever recovered, reaching a depth of 3,623 m (Petit et al. 1997, 1999). Ice cores are unique with their entrapped air inclusions enabling direct records of past changes in atmospheric trace-gas composition. Preliminary data indicate the Vostok ice-core record extends through four climate cycles, with ice slightly older than 400 kyr (Petit et al. 1997, 1999). Because air bubbles do not close at the surface of the ice sheet but only near the firn-ice transition (that is, at ~90 m below the surface at Vostok), the air extracted from the ice is younger than the surrounding ice (Barnola et al. 1991). Using semiempirical models of densification applied to past Vostok climate conditions, Barnola et al. (1991) reported that the age difference between air and ice may be ~6000 years during the coldest periods instead of ~4000 years, as previously assumed. Ice samples were cut with a bandsaw in a cold room (at about -15°C) as close as possible to the center of the core in order to avoid surface contamination (Barnola et al. 1983). Gas extraction and measurements were performed with the "Grenoble analytical setup," which involved crushing the ice sample (~40 g) under vacuum in a stainless steel container without melting it, expanding the gas released during the crushing in a pre-evacuated sampling loop, and analyzing the CO2 concentrations by gas chromatography (Barnola et al. 1983). The analytical system, except for the stainless steel container in which the ice was crushed, was calibrated for each ice sample measurement with a standard mixture of CO2 in nitrogen and oxygen. For further details on the experimental procedures and the dating of the successive ice layers at Vostok, see Barnola et al. (1987, 1991), Lorius et al. (1985), and Petit et al. (1999).