Silent witness of steamy days
(appeared in Aug 2017)

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Reptile fossils are helping map prehistoric temperatures of the Amazon, says S.Ananthanarayanan.

Powerful agencies have started working against the world’s resolve to contain global warming. A belief has grown that the earth has seen warm times in ages past and the results were not catastrophic. And a theory is about that the tropics, which account for 50% of the surface of the earth, would absorb heat and keep the earth’s temperature from getting extreme It is hence important to know how hot the earth did get during the warm phases of its geological history.

There are several indicators of how warm the earth has been at different times. The temperature range that has been worked out for the tropics, however, seems be too low, considering that the temperature at the poles at the time was like the present-day equatorial belt. The question was resolved some years ago by Prof Jason Head and a team of researchers in Canada and USA. The team described in the journal, Nature, a piece of evidence of considerably higher temperature in the tropics than had been estimated.

While temperature records have been kept only since a few centuries, we can identify older instances of very hot or cold years, or decades, from historical records or farming and trade documents. And reaching back, even thousands or millions of years ago, we can work out the temperature graph from remains of plants and animals that were common at different times in the history of the planet, and from geological features.

This collection of evidence has revealed a series of warm periods where the average annual temperature was some 6-7 degrees Centigrade above what we see today. This average, annual temperature in the 1950 – 1980 period was 14°C and 2015, a very hot year in recent times, saw a rise in the average by 1°C. We can imagine that the extremes at a time when the average rises by 6-7°C would be hot indeed! The trouble, however, is that this estimate of the average is not enough to explain the great increase in the temperature in the Arctic Circle during these warm periods. Fossil records show that there was lush vegetation, with crocodiles and palm trees in Wyoming and Siberia. These regions must have hence known temperatures like the equatorial belt, which is ‘natural’ home to crocs and palms trees. If the Arctic was thus some 30°C warmer than today, then the temperature in the tropics was certainly more than only 6-7 °C warmer than at present.

Reptile evidence

Prof Jason Head and his team reported a finding in Colombia, which is very near the equator, of a fossil of a snake, Titanoboa Carrejonensis, which was large as 13 metres long and weighed over a ton. Snakes belong the category of ‘cold blooded’ animals, whose bodies stay nearly at the temperature of their surroundings. This feature, of the body temperature being related to the environment, places constraints on how large an animal can get. The presence of giant snakes in Colombia thus becomes a proxy for the temperature at the time when the snakes were alive.

It was formerly thought that cold blooded animals had no means of controlling body temperature and were known as ‘poikilotherms’, a word that means ‘varied temperature’. The mechanism is now better understood and the preferred term is ‘ectotherm’, or animals that receive heat from outside, rather than generate heat from within. Cold blooded animals thus have a very frugal energy economy and they need to seek out places of optimum temperature to maintain their metabolism. We may have seen lizards sunning themselves on a rock and it is known that fish seek out the depths of water where the temperature is the most suitable. Insects are known to warm the muscles that are used in flight by rapid vibration or by focusing sunlight on the muscles. And cold blooded animals generally tone down their level of activity, even by hibernating, when the temperatures falls.

They are thus more dependant on the environment and, in challenging conditions, are likely to be driven to extinction by more adapted, warm blooded species. But, as they do not use energy to maintain body temperature, they need only 10 % of the nutrition that warm blooded animals need. They can hence drive the latter out where the conditions are right, like in tropical rain forests.

Size and temperature

While cold blooded animals can adapt metabolic rates according to available heat and even store energy for basic metabolism, there are different optimum conditions for animals of different sizes. Large ectotherms have greater body weight for their surface area. They are thus able to store heat. Sea turtles, for instance, can take in heat at the surface of the sea and make long dives into cooler, deep waters where the hunting is good. Conversely, the large body mass needs more energy for basic metabolism and large ectotherms cannot exist at all unless there is a minimum high temperature source of heat.

We can see that large ectotherms need a higher temperature environment from the fact that larger snakes on the earth are always found in warmer climates and cold countries are host only to smaller ectotherms. In fact, it is possible to link the size of cold blooded animals found in a region with the mean temperature and then to use information about animals in an unknown area to conjecture what the temperature may be.

Giant snake fossils

Prof Jason Head and colleagues used this relationship to derive from the snake fossil discovered in Columbia what the temperature may have been at the time the snakes were alive. The fossils date to 58-60 million years ago, which is in the heart of the last period of great warming of the earth. The temperatures that Prof Head and his team derive turns out to be 5-6° C higher than what other evidence has indicated.

They estimate an average annual global temperature of 32-34°C, which is a whole 17-19°C warmer than the 2015 average of 15°C. The average annual temperature at Nagpur, the geographical centre of India, is around 27°C. This year, Nagpur experienced a scorching maximum of 47°C. As any relief was welcome, it was ironic that Nagpurians rejoiced when the temperature was 45°C!

The levels of rise in temperature that are predicted if global warming continues unchecked would clearly drive familiar forms of life away from most of the parts of the earth that are now inhabited. Even if the Amazon teemed with vegetation and life during those steamy days, the Amazon got there through evolution over millions of years. In comparison, the earth’s present challenge is to adapt to a large rise in temperature within a human lifetime!

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