The rising oceans

As islanders, we should be worried about climate change. There is one aspect of climate change that we should be concerned about most: the rise in sea levels. The sea is likely to engulf at least certain parts of the world’s islands in the long term. Measurements tell us that the global average sea level is currently rising by about one inch per decade. It is not Day After Tomorrow or 2012 yet, but we are getting there.

This is a slow process, one which we will not essentially see in our lifetimes. In this invisible process, our long-term sea level rise commitment or “lock-in” - the sea level rise we don’t see now, but which carbon emissions and warming have locked in for later years - is growing 10 times faster, and this growth rate is accelerating. The entire process may take around 2,000 years.

An international team of scientists recently found that for every degree Fahrenheit of global warming due to carbon pollution, the global average sea level will rise by about 4.2 feet in the long run. When multiplied by the current rate of carbon emissions, and the best estimate of global temperature sensitivity to pollution, this translates to a long-term sea level rise commitment that is now growing at about one foot per decade.

That should be a serious cause of concern for all. Such rates, if sustained, would be the highest levels of sea level rise contemplated in hundreds of years. It can threaten the very existence of coastal communities everywhere.

In this scenario, there are two sea levels: the sea level of today, and the far higher sea level already being locked in for some future date. But here is the dangerous statistic: The amount of carbon pollution to date has already locked in more than four feet of sea level rise past today’s levels. That is enough, at high tide, to submerge more than half of today’s population in 316 coastal cities and towns in the lower 48 states of the United States alone. Extrapolate that all over the world and truly frightening picture emerges. By the end of this century, if global climate emissions continue to increase, that may lock in 23 feet of sea level rise.

Danger

The only way to prevent or mitigate this danger is reduce greenhouse emissions substantially, by almost threefold. Ideally, It includes a halt to global emissions growth by 2020, followed by rapid global emissions reductions, and a massive program to remove carbon from the atmosphere, resulting in net negative emissions - in effect, an atmospheric clean-up - by around 2100.

Even the rise in sea levels pales into insignificance when we consider the next threat - the oceans might boil away and vanish altogether. It is called the Venus Syndrome, whereby the Earth may one day end up just like Venus with an unbearably hot atmosphere.

Venus has a thick atmosphere that is 96.5 percent carbon dioxide, which keeps its surface at nearly 482°C. The planet's water boiled off long ago.

The question we have to answer is, could that really happen on Earth, which is farther from the sun, and where the CO2 level is rising past the 400 Parts Per Million threshold?

The scenario is not difficult to comprehend. As carbon dioxide warms the planet through the greenhouse effect, more water evaporates from the ocean - which amplifies the warming, because water vapour is a greenhouse gas too. Climate scientist James Hansen is among those who believe that fossil-fuel burning could cause the process to run out of control, vaporising the entire ocean and sterilising the planet.

In his book Storms of my Grandchildren, Hansen says : “If we burn all reserves of oil, gas, and coal, there is a substantial chance we will initiate the runaway greenhouse. If we also burn the tar sands and tar shale, I believe the Venus syndrome is a dead certainty.

There are scientists who oppose the Hansen model. They say that during the Paleocene-Eocene Thermal Maximum (PETM), 56 million years ago, a huge natural spike in CO2 sent temperatures on Earth soaring - but life went on and the ocean remained intact.

Nevertheless, physicists have been training supercomputers on the lowly water molecule, calculating its properties from first principles - and finding that it absorbs more radiation at more wavelengths. In a paper published in Nature Geosciences, those calculations have rippled into a simple climate model. The paper's conclusion: “The runaway greenhouse may be much easier to initiate than previously thought.” A frightening prospect indeed.

Radiation

The runaway greenhouse effect happens when the amount of incoming solar radiation exceeds a fixed limit. In simple terms, it happens when the Earth absorbs more sunlight than it can emit thermal radiation. The limit on how much radiation Earth can get out to space is smaller than previously thought.

Studies show that if we put about ten times as much carbon dioxide in the atmosphere as we would get from burning all the coal, oil, and gas - about 30,000 parts per million - then there could be a runaway greenhouse situation today.

It might not happen today, but there is no guarantee that it might not happen way into the future. As one scientist so succinctly puts it, “Venus’ past is Earth’s future”.

One other factor which may hasten the boiling away of the oceans is that the Sun increases its luminosity slowly with time. At the beginning of the solar system, the sun was only around 70 percent bright than it is now. It is going to be brighter in the future. Given that the runaway greenhouse happens when there's more solar radiation absorbed than the Earth can emit thermal radiation, oceans might be gone somewhere between half a billion and a billion years from now.

The good news is that we have a billion years to play around, to seek new ways of preventing that ultimate disaster. We can all do little things that reduce carbon emissions, like switching off an unwanted light bulb. And a billion years from now, Man would be sufficiently advanced to embark on interstellar voyages in search of second homes, even if their own world becomes a lifeless shell.