Remember Jim Hansen, the director of NASA’s Goddard Institute for Space Studies? He was the one that the Bush administration tried to silence after he gave a public warning that the current rate of fossil fuel use will make the earth into “a different planet.”

Hansen’s work is the subject of an article in Technology Review that explains how danegrously close we are to the point of no return. Look at this graph. It appears that every 100,000 years, small oscillations in the earth’s orbit cause minute changes in the amount of, and larger changes in the distribution of, sunlight on the earth. These changes caused natural fluctuactions in carbon dioxide levels, which in turn led to temperature fluctuations on the order of 5°C, enough to change ocean levels by 100 meters. It appears we are in the middle of such a natural fluctuation right now.

Now look at the very right edge of the graph. Carbon dioxide levels have skyrocketed since the start of the Industrial Revolution. This is clearly not part of any natural cycle– and is a harbinger of dire consequences:

Owing to greenhouse changes we have already incurred, Hansen told his audience in San Francisco, Earth’s temperature will rise about 0.5 ºC in the next 50 years even if we stop burning fossil fuels today. We’re on a slippery slope: we could cross a threshold that leads to a drastically different planet, half a century before knowing that we’ve done so. Hansen believes we are horrifyingly close to such a threshold, and that we will cross it if we don’t change our greenhouse ways within the next few years.

Earth is now passing upward through the highest temperatures of the past 12,000 years, and the half a degree that is already in the pipeline will bring temperatures within half a degree of the high points they have reached only a few times in the past two million years. During a warm period about 120,000 years ago, for example, sea levels were probably five or six meters higher than they are today.

Running future emissions scenarios on a GISS computer model, Hansen finds that if we remain on the path he calls “business as usual,” temperatures will rise between two and three degrees this century, making Earth as warm as it was about three million years ago, when the seas were between 15 and 35 meters higher than they are today. There go many major cities and the dwellings of about half a billion people.

The current issue of Technology Review is dedicated to the climate crisis and how the technologies exist to slow down human-induced climate change: time is running out, but it’s not too late yet.

Finally, to the skeptics who refuse to accept these conclusions and are happy to proceed with business as usual, I offer this: No one debates that modern technological advances, and in particular industrialization, change the environment (think strip mining, deforestation) and pollute the atmosphere (think smokestacks) and the oceans (think chemical effluvia). Surely all those byproducts will have some sort of effect, don’t you think? It’s not impossible that the scientists who have spent countless careers studying these phenomena could be wrong, but are you willing to take the chance that maybe, just maybe, they’re right? If we as a species clean up after ourselves and leave the natural world no worse than we found it, then, and only then, can we rest assured that climate fluctuations are not due to our activities.