As we in the northern hemisphere deal with the warmest part of the year, and many feel that, adding it all together, we may be enduring the hottest days ever recorded, let's cool down with thoughts of the much colder periods that may have prevailed in our world's prehistory.

In a November, 2003, essay, From Snowball Earth to Arctic Turf, I noted that there have, according to scientists, been such wide extremes of temperatures in the planet's past that at times it may have been almost completely iced over, both on land and sea, while at others even the poles were relatively warm and temperate.

Now there is new information available to the general public, in book form or online, about some of the wide variations that occurred millions of years ago.

What relevance such speculations or findings have for our modern situation is open to debate. Many believe that the comparatively rapid extent of global warming occurring at present could lead to a new radical shift out of the current climate balance, and that the "new world order" that develops from such a breakdown could be substantially hotter or colder, on average, than the fairly mild circumstances people came to take for granted in the first several thousand years of civilized human history. Others regard hypotheses of this sort as dramatic or alarmist, yet quite unlikely to have a bearing on our lives or those of many generations to come.

Whether or not one has made up her or his mind already on such questions, out of intellectual curiosity alone the latest info may be of interest. A little Google, or other major search engine, use can be rather fruitful. Here is a sample just on the overall issue of whether there may have been profoundly frozen epochs in Earth's history and, if so, how it may have affected life's development.

For the latest from the geologist who coined the term "snowball Earth," Joseph L. Kirschvink, you can avail yourself of info on his and his group's projects at the Cal Tech Kirschvink site. Among other things, he shows here the animated continental shifts about the time the Cambrian explosion occurred. Some think that Cambrian explosion of multi-cellular life forms was due to life's response to the relatively violent changes of the environment when the temperature rose or fell significantly.

Not everyone agrees with the all but totally frozen world theory (said to have enveloped the planet up to four times, each with average temperatures of minus 50°F or colder, but restored to conditions more to our liking after melting eventually followed periods of higher volcanic activity). For instance, at the NASA's Goddard Institute for Space Studies web site, one may find an article that discusses a "slushball Earth," when all would have been frozen except the oceans near the equator, where the waters would have stayed free of ice.

If all this seems a bit too over the top, why not instead just take a break from the heat with a quick dip in one of the northern states' refreshingly cool lakes, the still cold surf along our western US coast, or, in Florida, maybe a spring-fed Florida stream. Here in Austin, Barton Springs pool will hopefully be quite frosty enough to "chill" one's concerns over the very hot hot global warming summers projected to be in our future.

Source: Chill Out. Robert Anderson in Natural History, Vol. 114, No. 6, page 53; July-August 2005.