Now that we have already crossed above that level, the hope is simply that it may help awaken people and politicians to the dangers of runaway climate change in time for dramatic remedial action. If no improvement occurs in the current state of increasing carbon dioxide and methane being released into the atmosphere, there is a danger that, within the next few generations, possibly in this century, we shall not be able to contain the effects and that our descendants shall live in a world tens of degrees hotter, one with average temperatures likely to be far more alien than in the last century to human survival, though hospitable to tropical swamp creatures and to the easy sustenance of prolific diseases.

Less optimistic folks assume our species will maintain a head-in-sand outlook. They note that reporters and politicians with short-term perspectives cite a recent 15-year period, one in which overall warming has essentially not occurred, as reason to disbelieve the warnings, ignoring the explanation that the world's seas, by becoming saturated with excess carbon dioxide, have temporarily taken up some of the slack in the balance that supports our biosphere. Just as a glass of water can only absorb so much Alka-Seltzer, the oceans though have a limited capacity for dissolving further CO2, and once that ceiling is attained Earth could see a rapid rise in the mercury readings.

There are those who believe human behavior is simply not very subject to change and that growing populations, accompanied by rising economic growth and industrialization in developed and third world nations alike, make runaway climate change almost inevitable. They have begun to call for geoengineering remedies. The idea here is that our species has a genius for technological solutions, so why not apply that talent to what may be our biggest challenge yet, the rising temperatures associated with our aggressive domination of the planet?

Miocene Fauna (Wikipedia)

Indeed, some say we ought to nip the problem in the bud and very soon take steps needed to reflect into space some of the solar radiation that, through the greenhouse effect, is causing so much difficulty, with currently more energy coming in than is being released back out. Various proposals have been made for how this might be done, including adding sulfur dioxide to the upper atmosphere, putting reflective pieces of plastic into orbit between us and the sun, whitening clouds, etc. In fact, it is not, they say, a question of whether or not to geoengineer, but rather to rein in what is now metaphorically an out of control horse, the unintended consequences of the geoengineering we have been doing anyway, the ways we have already been reshaping the planet, yet without much awareness or forethought, through raising atmospheric CO2 levels and so initiating a period of global warming.

What are pros and cons of attempting to become Earth's stewards on this vast new scale, trying to manage all the delicate balances of nature ourselves?

On the plus side of the ledger, if we do not correct the damage that has already been done and is continuing, then, as mentioned above, our planet's average surface temperature may rise to that of the Congo. In addition, if we are successful, geoengineering-type actions might give mankind enough extra time to wean ourselves of an addiction to carbon-based fuels and processes. The costs, though large, should be far less than if nothing is done and we enter a new phase in which rising temperatures and their effects truly get beyond our ability to manage.

The sobering dark side of the geoengineering debate is that we actually do not know what we are doing, that the intricate system which balances our world's air, water, and land surfaces is far more complex than we imagine and that if we tinker with something in order to fix it we risk ruining it instead, as surely as if we had taken a hammer to the best of old analog Swiss watches. Even as wars are only predictable up to the point they commence and then almost inevitably take new, unanticipated turns, once we begin the project of managing Earth we shall likely be engaged in a wild ride on an untamed steed. Suppose we cannot safely dismount once in the saddle? With the repercussions so huge if we miscalculate, perhaps a bit of humility is in order from the outset. It has been said that it is far easier to prevent Humpty-Dumpty's fall than to put Humpty-Dumpty back together again.

Nonetheless, supposing that, for those with the means, the pro arguments outweigh the con debates, what might be done in the next few years or decades to save a relatively temperate planet Earth we know and love?

1. Sulfur Dioxide or Other Aerosols - Injections of stratospheric gases via aircraft, missiles, or balloons would block solar radiation from reaching the surface of Earth.

2. Dusting the Upper Atmosphere - Ordinary soil dust would be more benign than sulfur aerosols. An estimated kg of dust would need to be deposited in the upper atmosphere to offset the effects of each ton of carbon emissions. As the dust would eventually be rained back to the surface, it would have to be replenished.

3. Cloud Reflectivity Enhancement - Large fleets of robot ships would spray fine mists of seawater into the atmosphere to thicken clouds and so increase their reflectivity.

4. Tree Removal and Addition - Trees would be removed from snowy mountain areas, increasing the regions' reflectivity (deforestation). Trees would be added to tropical areas (reforestation). This has been found to have a net global cooling effect.

5. Cool Roof Technologies - Roof surface construction would be chosen for its relative reflectivity. In addition, roof areas would be painted white or pale colors.

6. Reflective balloons - Billions of aluminized balloons in the upper atmosphere could reflect enough solar radiation to offset the effects of global warming. However, their cost is estimated to be 20 times as great as placing enough dust into the stratosphere.

7. Mirrors in Space - Some have suggested putting millions or billions of pieces of reflective plastic into orbit between us and the Sun. Others have pointed out that similar or even superior results could be obtained from orbiting a concave grid of thin wire mesh, 1000 kilometers wide and a few km thick, at a key point between Earth and the sun. Solar wind might push this diffraction grid out of the optimal location. However, if a solution to that problem were found, this theoretical fix might be reasonably cheap.

With the unfortunate landmark of 400 ppm of carbon dioxide in the atmosphere now having been achieved, it is vital that corrective measures be taken to prevent a much worse milestone, severe long-term global warming effects. A broad public discussion is needed along with concerted political action to determine the best solutions. If we lack the will in the reasonably near future to cease further global warming gases pollution of the atmosphere, we may have no rational choice but to use geoengineering techniques to buy ourselves more time.

Primary Sources:

-Solar Radiation Management in Wikipedia; last updated January 17, 2014;

-The Last Time CO2 Was This High, Humans Didn't Exist. Andrew Freedman in Climate Central; May 3, 2013.