17 June 2010
How Do Global Warming, Greenhouse Gases And The Earth's Orbit All Link Together?
I've written a number of blogs now where I try to get to grips with some of the numbers underlying global warming. So far, I agree that the historic figures do demonstrate global warming over the last 100+ years within a relatively wide band of possible growth rates, and that the evidence shows that the levels of carbon dioxide have increased since about 1950. Furthermore, the science behind the link between carbon dioxide, methane and the greenhouse effect (and so global warming) is simple science that has been known for many years; in fact, global warming/the greenhouse effect is science that enables life on earth as without it our planet would have an inhospitable temperature closer to -18oC, i.e. it heats up the earth by about 33oC already to 15oC.
What interests me next is how have temperatures and greenhouse gases moved in the past? I will try and analyse this by looking at some neat science that analyses climate data over a longer period. This will get to the nub of the issue, i.e. what is driving global warming, plus may answer my two current quandaries: (i) why isn't global warming being driven exponentially by the very obvious and definite growth in CO2 per the Mauna Loa graphs? (ii) why do the predictive models of climate scientists suggest that we should be preparing for increases of 2 - 4oC for the next 100 years, while history shows global warming is more like 1oC over the last 100 or so years?
To work this out, some scientists look at what happened at the end of earlier ice ages as this hints at the mechanics of atmospheric and other climatic changes. There are several pieces of research that suggest how temperature and greenhouse gases interact - one strand looks at climate data in stalagmites in caves in China and the other is a series of classic pieces of climate change science around ice core data from the Antarctic. I'll deal with the Antarctic first.
The first one is pretty neat. It is based on the fact that when snow falls it traps air in small bubbles within its structure. Then as more snow falls the next year, this new layer not only brings its own store of information about air quality, snowfall, temperature and levels of greenhouse gases, but it permanently seals off the information stored in the previous year's snowfall. Over time, we get left with an annual layering of data that goes back for ages and ages in the Antarctic, as well as in the Arctic especially on Greenland. Scientists have now dug vertical small circular shafts into the ice and then, after chopping up these ice cores, have analysed the information from them - there's a video on Youtube that shows you what the scientists do. Data collated from other similar projects basically corroborates information found in this much earlier paper, which was published back in 1999 by Petit et al (detailed reference at end).
In essence, Petit et al were able to drill down 3,130m, covering 420,000 years and providing a climate record through four climate cycles. They found that temperatures are constantly changing, but always within given maximum and minimum levels. They found that when concentrations of greenhouse gases (specifically CO2 and CH4) in the atmosphere went up, global temperatures, also, went up and vice versa. They concluded "[f]inally, CO2 and CH4 concentrations are strongly correlated with Antarctic temperatures; this is because, overall, our results support the idea that greenhouse gases have contributed significantly to glacial-interglacial change. This correlation, together with the uniquely elevated concentrations of these gases today, is of relevance with respect to the continuing debate on the future of earth's climate." (Petit et al, 1999).
However, they also stated earlier that "[t]hese results suggest that the same sequence of climate forcing operated during each termination [of a glacial period]: orbital forcing (with a possible contribution of local insolation changes) followed by two strong amplifiers, greenhouse gases acting first, then deglaciation and ice-albedo feedback." (Petit et al, 1999). This suggests to me that greenhouse gases, the melting of the ice caps and the positive feedback caused by white ice turning to dark seas usually act as amplifiers of changes in temperatures caused by other factors such as changes in solar energy caused by changes to the earth's orbit, which gets me back to one of my original quandaries - what is driving climate change and so what happens when you have the amplifiers without necessarily the increased temperatures resulting from either a more active sun or a change in the earth's orbit?
The next paper I read was in New Scientist only a couple of weeks ago and is also pretty cool. This work is trying to understand the end of glacial periods by analysing stalagmites in caves in China and interlink this with known changes in the shape of the earth's orbit - now how amazing is that? Since then, I have been reading the original scientific papers, hence the time delay in writing this blog.
Firstly, we need to start with the concept of Milankovitch's theories on the earth's orbits. Milutin Milankovitch undertook detailed calculations on the earth's three main orbital cycles. So, for example, every 41,000 years the tilt of the earth's axis increases and decreases, making summers hotter and colder respectively. It's summer temperatures that are important as this is what drives the potential for ice packs to melt over time, rather than winter temperatures which just create more ice. So from 2.5 million years ago to about 1 million years ago the ice ages occurred based on these cycles. However, around 1 million years ago to the end of the last ice age, glacial periods started occurring every 100,000ish years. This links in potentially with another orbital cycle of 95,000 - 125,000 years, but here the science is less strong and debate still rages as to what is actually happenning.
Liu et al have measured oxygen-18 in stalagmites in several caves in China. Water containing oxygen-18 is heavier than normal oxygen-16 and so condenses more easily, so heavy monsoonal air loses much of its oxygen-18 as it moves inland and each year a record is left on the stalagmites. As each glacial period ends, the summer monsoons became much weaker than normally and so the oxygen-18 levels in stalagmites increased. Their evidence showed that monsoons failed in the last four glacial terminations, or as they write "[t]his climate pattern, broadly resembling other cave records from China, appears to correlate with multi-decadal to millennial changes in Greenland temperature and the general pattern of the wind-borne calcium ion record in the ice." In fact, work on a stalagmite from the Dongge Cave in China agrees exactly (within error) with the Vostok ice core records of Petit et al, showing methane rise in the atmosphere at 129,000BP.
Further work has shown that CO2 and CH4 levels increase at the same time as the ice packs at the poles decrease, suggesting that the reduction in ice is actually causing the rise in CO2 and CH4. It is suggested that as the tilt of the earth's axis changes this increases the temperature of the earth and the ice sheet over North America flows into the Antarctic, which interferes with and then stops the circulation of water around the oceans, which normally keeps the southern hemisphere warmer and the northern cooler (the so-called Atlantic Meridional Overturning Circulation). As the southern seas warm up, CO2 is released into the atmosphere as CO2 is less soluble in warm water than cold and so further increasing the impact of the higher temperatures from the sun. In effect, over the last 400,000 years, whenever the tilt of the earth's axis reached a maximum, the intensity of sunshine increased (based on insolation in July at 60o north), CO2 levels increased to a maximum, relative sea levels also increased to maxima, all correlating with the strength of the Asian monsoon.
All of this comes from ice core data, analysis of stalagmites and other stores of climate data like coral reefs. That in itself is amazing. Then there is agreement in climate records going back many hundreds of thousands of years that correlate with each other across the world and using different techniques and types of ancient, geological record.
Finally, I would like briefly mention another set of amazing work by Zachos et al in the US which tries to get to grips with temperature and atmospheric gases in deeper time in the order of millions of years ago. They have analysed various types of proxy data in deep sea cores of rocks to determine temperature and carbon in earth's history and have tried to relate this to events in the geology of the earth and evolution, so developing a framework for the development of the earth's climate over a much longer timescale. What I like about these pieces of research is not just how clever they are, but also because Bjørn Lomborg uses them in a section trying to refute the science within climate change work in his book "Cool it - the skeptical environmentalist's guide to global warming" - he has a tendency to misquote, or at least to quote out of context, as well as jumble up numbers and data to make his own points, which are often at odds with what the original scientists actually have stated. Just like Nigel Lawson, Bjørn Lomborg has authority when it comes to economic and political discussions around climate change, but they sometimes get it wrong when they try and refute the science; most of their errors stem from two simple problems: (i) they don't understand the scientific process; and (ii) they mistake/confuse weather for climate. I will try and get back to Lomborg's book "Cool It" some time and show how he shoots himself in the foot at times by blatantly altering the available research to suit his arguments.
The research by Zachos et al shows that carbon and temperature are correlated at least to about 34 million years ago at the edge of the Eocene and Oligocene Ages, which is when the Antarctic Ice-sheets became fairly permanent, and that there is correlation with the orbit of the earth around the sun even if the impact is sometimes relatively weak over millions of years. Prior to then, getting clarity in the temperature and carbon dioxide levels gets ever harder and we find that the linkage between carbon levels and temperature is much less clear and even perhaps non-existent, however later research by Zachos et al indicates that this lack of correlation may not be as extreme as some researchers have indicated and is perhaps simply a result of lack of experimental data. The other interesting occurrence is that whenever there has been a sudden change in temperature this has also been accompanied by a similarly sudden change in carbon; these occurred at 23, 34 and 55 million years ago. Later research at the 34 million years ago tipping point suggests that carbon dioxide is a key factor in climate transition; Pagani et al (2005) showed that "[i]n detail, a trend toward lower CO2 concentrations during the middle to late Eocene, reaching levels at the E[ocene]/O[ligocene] boundary that could have triggered the rapid expansion of ice on east Antarctica; and work by Pearson et al (2009) indicates that there was a fall in atmospheric carbon dioxide at 34 million years ago that triggered climate transition to an ice-house world and "[t]his study reaffirms the links between cryosphere development and atmospheric carbon dioxide levels at the largest and most important climatic tipping point of the last 65 million years."
However, when you look at research into climate over such a long time period, you realise pretty quickly that long term climate progression is the sum of many different processes and that it is far more complex than any of the commentators and scientists would have everyone believe, plus that correlation does not necessarily mean causation.
In overview, we know that the earth's general temperature, hence climate, has gone up and down over time dependent on the earth's tilt and orbital shape, i.e. effectively how close the earth gets to the sun during its orbit and so how much solar energy gets to the earth. These changes in temperatures are then further affected by the earth's environment, especially the levels of greenhouse gases in the earth's atmosphere, the ice sheets and the ice-albedo effect. In addition, climate gets impacted by a whole raft of other factors ranging from geological through to biological, which is a point that I will get back to in a later blog.
The science, therefore, does show that the basic greenhouse effect has impacted climate in the earth's past and present and so will affect it in the future, but that it is not the only factor that impacts climate nor perhaps the most important climate factor over longer time periods. Furthermore, while the research does indicate that sudden changes in carbon dioxide often occur with quick moves in climate, it doesn't explain the consequences of these amplification or forcing impacts on our future climate, so that's my next journey and is where I will need to start investigating the computer models devised by climate scientists to predict the climate in the future.
Before I go there, however, I would like to round off this section of my journey around global warming /climate change with a look at some of the other indicators of current global warming, such as sea levels and sea acidity just to round off the historical and current status of climate indicators.
References
Battersby, S. (2010) Meltdown: Why ice ages don't last forever, New Scientist, issue 2761, 24 May 2010, Available on the Internet at http://www.newscientist.com/article/mg20627610.900-meltdown-why-ice-ages-dont-last-forever.html (Accessed 25 May 2010)
Kelly, M. J., Edwards, R.L., Cheng, H., Yuan, D., Cai, Y., Zhang, M., Lin, Y., An, Z. (2005) High resolution characterization of the Asian Monsoon between 146,000 and 99,000 B.P. from Dongge Cave, China and global correlation of events surrounding Termination II, Palaeogeography, Palaeoclimatology, Palaeoecology 236 (2006), 20 -38, Available from the Internet at http://www.sciencedirect.com.libezproxy.open.ac.uk/science?_ob=MImg&_imagekey=B6V6R-4JX38YR-1-R&_cdi=5821&_user=126980&_pii=S0031018206001301&_orig=search&_coverDate=06%2F23%2F2006&_sk=997639998&view=c&wchp=dGLzVlb-zSkzV&md5=a8ffdf76ab6ec0bde843ad331aeaa780&ie=/sdarticle.pdf (Accessed 2 June 2010)
Liu, D., Wang, Y., Cheng, H., Edwards, R.L., Kong, X., Wang, X., Hardt, B., Wu, J., Chen, S., Jiang, X., He, Y., Dong, J., Zhao, K. (2010) Sun-millennial variability of Asian monsoon intensity during the early MIS 3 and its analogue to the ice age terminations, Quaternary Science Reviews 29 2010, 1107 - 1115, Available on the Internet at http://www.sciencedirect.com.libezproxy.open.ac.uk/science?_ob=MImg&_imagekey=B6VBC-4YHSCPG-1-B&_cdi=5923&_user=126980&_pii=S0277379110000107&_orig=search&_coverDate=05%2F31%2F2010&_sk=999709990&view=c&wchp=dGLzVlz-zSkWb&md5=2e3a05cdb40b477b6a09872b4120f444&ie=/sdarticle.pdf (Accessed 25 May 2010)
Pagani, M., Zachos, J.C., Freeman, K.H., Tipple, B., Boahty, S. (2005) Marked Decline in Atmospheric Carbon Dioxide Concentrations During the Paleogene, Science 309, 600, 22 July 2005, Available on the Internet from www.sciencemag.org (Accessed 7 June 2010)
Pearson, P.N., Foster, G.L., Wade, B.S. (2009) Atmospheric carbon dioxide through the Eocene-Ologocene climate transition, Nature 461, 1110- 1114, 22 October 2009 Available on the Internet from www.nature.com (Accessed 7 June 2010)
Petit, J.R., Jouzel, J., Raynaud, D., Barkov, N.I., Barnola, J-M., Basile, I., Bender, M., Chappellaz, J., Davis, M., Delaygue, G., Delmotte, M., Kotlyakov, V.M., Legrand, M., Lipenkov, V.Y., Lorius, C., Pepin, L., Ritz, C., Saltzman, E., Stievenard, M. (1999) Climate and atmospheric history of the past 420,000 years from the Vostok ice core, Antarctica, Nature, Vol 399, 3 June 1999, Available on the Internet from http://www.daycreek.com/dc/images/1999.pdf (Accessed 25 May 2010)
Zachos, J., Pagani, M., Sloan, L., Thomas, E., Billups, K. (2001) Trends, Rhythms, and Aberrations in Global Climate 65 Ma to Present, Science 292, 686, 27 April 2001, Available on the Internet from www.sciencemag.org (Accessed 7 June 2010)
