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THE CLIMATE AT THE END OF THE 20TH CENTURY

Human Influence Without Control?

Introduction

For the first time in history, human activities are able to influence the world climate to such an extent that these effects are becoming noticeable, i.e. so large that they start to exceed the 'natural noise'.

The temperature on Earth rose by 0.3 C between 1900 and 1940. This was mainly attributable to Solar activity. This was also the case for the slight cooling of the climate during the 1950s and 1960s. This is connected with an 80 year cycle in Solar activity discovered by Lassen and Friis Christensen.

In this period the heating effect of CO2 was counteracted by the cooling effect of SO2 (sulphuric aerosols - 'The Human Volcano'). After this the greenhouse gas contribution due to human activities 'kicked in'.

From 1978 on a sudden temperature jump has been observed. The temperature has been rising at a rate of about 0.18 C per decade since then. According to various studies about 0.1 C of this can be attributed to human activities ('the anthropogenic signal').

In the graph (source: van Dorland, KNMI - The Dutch Meteorological Institution) given below the various contributions with respect to the world average temperature change are given.

temperature on Earth

Some causes of natural variations are:
  1. The behaviour of 'the weather machinery' (oceans, atmosphere) itself. Think of: the North Atlantic Oscillation, El Nino, Blockades in the mid latitudes ... etc.
  2. Solar activity
  3. Volcanism, e.g. El Chichon (1983) and Pinatubo (1991)
A question which arises is: What are the possible consequences apart from the results of various model studies which are no longer disputed (such as a rise in sea level of at least 0.2 m and a temperature rise of at least 2 C by the year 2100)?

First one can think of natural disasters. One can also think of more indirect but potentially far reaching effects connected with a possible non-linear response of 'the weather machinery': A stepwise climate change.

Natural disasters

Recently quite some stories have been made public in which it is stated that the number of natural disasters has been increasing in the last decade. Many of these stories are of doubtful value.

When looking at the hard statistics very often no correlations can be found.

E.g.:
The number of hurricanes/typhoons in both the Atlantic and East- and West Pacific has not been increasing - neither in numbers, nor in strength. If one considers major (cat. 3 or stronger) landfalling hurricanes in the USA (good records since 1886) per decade, the 1990s (4) do not stand out. The 1940s (10) and 1950s (8) had more 'hits'.
(see: http://www.euronet.nl/users/e_wesker/atlhur.html)

It is the increased population combined with changed land use which quite often aggravates the impact of natural disasters. For example, when Hattie (1961), a hurricane of the same category as Mitch, made landfall in Belize/Honduras, it was no all out disaster like in 1998.

Secondly, older disasters are forgotten or not known by many. Two examples:

On 1 September 1923 a typhoon made landfall near Tokyo. A storm surge made its way into the Bay of Tokyo. Seismic faults stressed to their limits yielded at this very moment. The ensuing earthquake caused fires (similar to the 1989 San Francisco quake) which were whipped up to a firestorm by the incoming typhoon. Over 100.000 people lost their lives.

In November 1970 a strong cat. IV cyclone made a straight hit into the Ganges-Brahmaputra delta, sending a massive 20 ft wall of water into it. It became the worst storm surge disaster of the century. The toll (estimation in H.H. Lamb's 'Climate, History and the Modern World') was approximately 750.000.

Although there are some numerical studies which indicate that hurricanes and other types of storms might become more intense in future, this is still an unresolved issue.

Indirect effects

Subjects which are objects of recent research are, amongst others, the North Atlantic drift and the ice cover of the Arctic Ocean.

Recent publications in 'New Scientist' and 'Nature' point to the possibility of a repeat of the Yonger Dryas cold wave scenario. In this scenario a freshening of the North Atlantic, due to meltwater from glaciers and increased rainfall, slows down the overturning and descent of (cold and salty) water in this part the Atlantic. As a result, the North Atlantic drift slows down, which leads to a drastic temperature drop in Europe, Greenland, the North Atlantic and the adjacent parts of the Arctic Ocean. In a recent publication in New Scientist, results from research on deep water flows in the North Atlantic showed that the abyssal 'return current' from the Greenland Sea to the Norwegian Sea had stopped. This was in line with model calculations on a shut-down scenario of the North Atlantic drift. In 'Nature', a publication was made in which it was shown that initially the tropical part of the Atlantic became warmer due to the slow-down / shut-down of the North Atlantic drift. What makes these two publications notable is that they no longer deal with 'models only' material.

With respect to the two publications mentioned above, a seemingly contradictory publication was made in 'Science'. Based on satellite and surface observation, the conclusion is reached that the ice cover of the Arctic Ocean has diminished in surface area. Subsequently, simulations were made using the Geophysics Fluid Dynamics Laboratory and Hadley Centre climate models, to see whether these observations could be explained by natural variability. The chance that this was the case was only 2% for the 1978-1998 data and less than 0.1% for the 1953-1998 sea ice trends. In 'Geophysics Research Letters', the results of ice thickness measurements were presented. Sea ice thickness decreases of 37-55% have been observed when comparing data from 1958-1976 with 1993-1997.

These two stories tell us that the temperature in the North Atlantic and its adjacent parts of the World can go either way: quickly up - due to decrease in ice cover; or quickly down - due to slow-down / shut-down of the North Atlantic Drift. Research in this intriguing field will go on. Many issues are still unresolved. However, it shows that there are possible non-linear responses of 'the weather machinery' which a couple of decades ago were hardly imaginable. (Postscript 1/8/2006: Recent understanding assesses the possibility of a cooling scenario as remote)

Influence without control

If there were a 'Manual for Planet Earth', the chapter on climate regulation would probably read: 'The climate is tuned to maximum comfort for mankind. It is strongly advised to keep your hands away from the control buttons!' The big question is: 'Do we keep our hands away from these buttons?' The last evidence from research on climate change related subjects starts to show that this is at least to some extent not the case.

To use the metaphor of a steering wheel: Human activities do 'steer' a bit, but nobody knows what the wheels in front will do. Will we go left, straight on, or right? Will the climate on Earth, while warming a bit, stay relatively stable, like it did in the past 10.000 years, after it had been switched in a sort of roller coaster mode for over 100.000 years, or is it in the vicinity of some critical threshold towards a step change? Nobody knows.

I think that this non-quantifiable risk is much more important when calling for caution (with respect to changing the chemical composition of the atmosphere due to human activities) than hypes about natural disasters. These are bound to happen sooner or later anyway and maybe even irrespective of global warming.

This is also part of the problem for policy makers. Politicians often only tend to react after (big) incidents.

Evert Wesker, 16/12/1999 (small addition 1/8/2006)


References:


1. New Scientist 27 November 1999 referring to the following sources:
   Deep-Sea Research (vol 46, p 1), Journal of Climate (vol 12, p 3297)
 
2. Authors:      C Ruhlemann, S Mulitza, PJ Muller, G Wefer, R Zahn
   Title:        Warming of the tropical Atlantic Ocean and slowdown of
                 thermohaline circulation during the last deglaciation
   Full source:  Nature, 1999, Vol 402, Iss 6761, pp 511-514
 
3. Authors:      KY Vinnikov, A Robock, RJ Stouffer, JE Walsh, CL Parkinson,
                 DJ Cavalieri, JFB Mitchell, D Garrett, VF Zakharov
   Title:        Global warming and Northern Hemisphere sea ice extent
   Full source:  Science, 1999, Vol 286, Iss 5446, pp 1934-1937

4. Authors:      DA Rothrock, Y Yu, and GA Maykut
   Title:        Thinning of the Arctic Sea-Ice Cover
   Full source:  Geophysical Research Letters, 1999, Vol. 26, No. 23, pp 3469-3472


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