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Seminar reminder and Discussion Thread.
Friday 22 August: Can we distinguish between natural and human- induced climate change?
It’s true there are lots of factors that can contribute to a warming planet, and it’s also true that the earth’s climates have been all over the map (literally!) during its 4.6 billion year existence. We hear about solar cycles, volcanoes, aerosols, even cosmic rays causing or offsetting the warming that’s been documented during the last century. But the most plausible culprit is also the one that speaks best in its own defence. That’s right. Us. Humans are amazing creatures, capable of incredible feats, one of which has been shown by scientists to be altering the climate of a planet. In this seminar, Prof Brook explains how we know this to be true.
In this second of the Climate Q&A series, Prof Brook helps us sort through what’s true and false regarding the drivers of climate change, past and present.
Some of the sceptical positions to be covered include:
- Climate’s changed before
- CO2 lags temperature
- There is a natural climate cycle
- It warmed before 1940 when CO2 was low
- It cooled mid-century
- Other planets are warming
- Natural emissions dwarf human emissions
Climate change is primarily caused by…
- the sun
- solar cycles
- cosmic rays
- the ocean
- water vapour
- aerosols
- volcanoes (or lack thereof)
- the ozone layer
Guest Speaker: Professor Bob Hill, Executive Dean, Faculty of Sciences, University of Adelaide.
Bob’s botanical research has made significant contributions to the areas of palaeobotany, plant systematics, plant ecophysiology and the application of research from these areas to interpreting changes that have occurred to the Australian flora through evolutionary time. He he has won many awards including the Clarke and Burbidge Medals for his research into the impact of long-term climate change on the evolution of Australian vegetation.
Time and place:
5.30 -7.00 pm, Friday 22 August 2008
Lecture Theatre 102, Napier Building, University of Adelaide (North Terrace Campus, City)
Any questions or comments, ask away!
Filed under: Clim Ch Q&A, Sceptics
Dear Prof. Brook,
You described in friday’s presentation {22nd August, lecture 2} the dynamics of how solar radiation becomes long wave radiation {LWR} and that this heat interacts with GHG molecules to induce further heating in the atmosphere. I am waiting to see if the slides from that presentation become available because there is one conundrum that this simple model has for me – the fact the infrared radiation is not heat as you put it, but a form of electromagnetic energy which by itself is “cool” energy. It does have a high propensity to induce heating when it collides with other molecules and with this in mind I can see how it is easy to associate IR with heat, but by itself it is not heat, it is potential heat.
Our incoming solar radiation originates from the sun with a surface temperature near 6000 C. This produces a continuous spectrum of energy which has at the low end Ultraviolet, in the middle visible and at the high end infrared radiation. You showed an energy curve in your presentation which shows the top hat curve of incoming solar radiation and a similar one for outgoing radiation {LWR} that originates from surfaces on earth. Black body radiation theory predicts such a curve based on temperature and the incoming spectrum nearly matches this curve, except where absorption within the atmosphere punches out parts of the spectrum.
It is easy to confuse infrared radiation with heat because most of the time when we experience it is when we are witnessing its conversion into heat. An analogy is with electricity which is a form of energy that mostly ends up as heat. But, despite its ultimate destiny, you cannot call electricity “heat” because it clearly is not. The same applies to infrared radiation which is not heat, but a form of electromagnetic radiation with all the characteristics of EMR. I think the distinction is important.
Circumstantial evidence to support the “coolness” of IR is all around us. 50% of solar radiation is IR and space is bathed in it, but the temperature of space remains a cool 4K. Turn on an IR lamp on a cold morning and expose it to your naked skin and you “feel” instantly warm. Turn off the IR lamp and you “feel” cold again. The air temperature has not changed throughout this episode, but your skin has been warmed by induction – IR striking your skin and converts the IR into heat {molecular heat of vibration} at your skin’s surface.
Glass demonstrates another feature of IR and your example of a greenhouse almost described this. Sunlight striking glass does several things. UV does not penetrate glass, that’s why we don’t get a sun tan if we live indoors. Visible light freely passes through glass. IR does not pass through glass as it is absorbed by the medium. What happens is the energy bound in IR is converted into heat within the substance of the glass and this causes the glass to get quite hot, even hotter than the ambient air because glass {like metals} has a high conductivity and a low emittance. The combination means more and more energy can be poured into the substance and its temperature rises as the IR is converted into molecular heat.
That is why the trend of modern buildings with large panes of glass exposed to sunlight is problematic. Most people think it is the heat coming from outside that makes the glass so hot. That is wrong, it is the heating of the glass itself by the conversion of IR into heat that makes glass hotter than the surrounding air. If you think about it, it has to be true based on the laws of thermodynamics alone, one of which states that energy cannot by itself move from a lower to a higher level. How else can you explain how a sheet of glass exposed to sunlight ends up being hotter than the surrounding air?
The warm glass now starts to lose its heat in a few ways. Conduction throughout the body of the glass distributes the heat evenly throughout its substance. Conduction to the air adjacent to the glass get releases some of the energy. Convection may form where a body of air is sufficiently warmed to reduce its density compared to surrounding air and the less dense body of air rises, carrying with it the energy of molecular heat. The third form of release of energy is called “Radiation” and that is handled by IR.
All bodies above zero K emit IR in a fashion described in the theory of black body radiation and the warm glass is no exception. It emits IR in all directions, both into the surrounding air and into the glasshouse. This secondary IR then strikes surfaces within the glasshouse which turns the energy into heat and this in turn releases the several forms of heat as well as its own IR etc etc. As a result, the air temperature inside the glasshouse heats up.
If IR was heat, then glaciers would not exist. The heat from the sun would melt them. Such is not the case despite glaciers being exposed to sunlight in some cases for up to 20 hours a day in high latitudes during summer. The reason for them melting it warm air, rainfall and secondary heating of rocks and soil that falls onto their surfaces being warmed by solar radiation and in turn melting as a consequence. Look closely at the surface of a glacier and surrounding a black rock, the ice has receded much further than in a clear white field. In clear spaces, the ice has survived solar radiation exposure for hundreds of years.
Our earth loses energy in the form of IR as it radiates into the atmosphere and then out into space. We do not lose “heat” to space. We cannot lose heat to space because space is a vacuum. Heat does not pass through a vacuum {eg the functionality of a thermos flask} because in the absence of molecules, the propogation of vibrating molecules is not possible. The molecules that make up our atmosphere are staying with us {thankfully} and therefore we cannot lose heat into space in this manner. We lose cold EMR in the form of IR into space.
The point I am trying to make is that IR is not heat per se, but a potent source of molecular heat. I know it is called “radiant heat” but that term confuses the physics because it is not heat at all, merely a means of carrying EMR over great distances to induce heat remotely. You mentioned in your talk the temperature behaviour of the moon, which during sunlight exposure had surface temperatures of over 100 C and at night the temperatures quickly plummet to minus 100 C. Surely, isn’t this a good example of the true nature of IR at work?
You may ask why I am trying to make this point?
The distinction is important to unlock a second treatment modality available to us to overcome the effects of GHG and the greenhouse warming. What I believe we have missed is the fact that the conversion of incoming SWR into LWR is not fixed and invariable, but is instead highly variable and alterable. We could engineer surfaces that currently take SWR and convert it into high levels of LWR that instead work as reflectors of SWR. This would bounce straight out of our atmosphere the SWR that has already been fully exploited by the atmosphere on the way in and would not contribute further to Earth’s heat balance.
If we use this to plan to reduce the surfaces that convert SWR into LWR on a macro scale, then we can effect earth’s heat balance mechanism in the presence of high levels of GHG’s. Landscape management is resulting in subtle changes to the daytime temperature that surfaces reach and along with this causing a shift towards the hot end of the spectrum and along with this a different LWR profile compared to cooler surfaces. This I also believe has a bearing on the global heat balance.
Our oceans cover the majority of Earth’s surface and most of the energy impinging on it is absorbed and transported around as the specific heat of water which is second only to ammonia in heat capacity. It determines to a major extent the weather we experience and by geoengineering and intercepting solar radiation that would end up heating our oceans we have a tool to alter the heat balance despite a background of increasing GHG levels.
What is exciting is that the possiblity exists for us to install the geoengineering in such a way that specifically targets regions which are disadvantaged by climate to alter their dynamics so that the resultant weather is more in keeping with our aspirations. For instance, by reducing the heat engine emanating from the core of our continent, it may be possible to weaken the predominant high pressure cell over continental Australia and encourage the northward movement of cold frontal systems.
We could increase evaporation over seas in sub-tropical zones by converting solar radiation into greater warming of the surface skin and as a result put more water vapour into this air which due to other causes is predominantly dry. This may contribute to increases in rainfall in sub-tropical regions.
In tropical zones we could reduce the intense heating that triggers the great Hadley cells that are the ultimate cause of deserts. The mechanism is simple: hot moist air rises due to convection in the tropics and thunderstorms over the tropical zone remove water vapour from the air. Thunderstorms release large amounts of potential energy {latent heat of evaporation} which results in high atmosphere being dry and hot. The warm high air moves poleward near the stratopause and continues to around 30~35 degrees from the equator when it starts to descends to the ground. As is falls, increasing pressure makes the air even hotter and drier, producing the predominant dry blast that results in deserts. If you take the driver of this cycle away by reducing the intense heating of land and water in the tropical region we will weaken the Hadley cycle and in turn this will push the influence of frontal systems equatorward.
The beauty of this approach is that whilst expensive, it will probably end up being cheaper than our attempts to tackle the problem by reducing carbon emissions alone. It will give targeted benefits to the people who install the system by altering local and regional weather.