But what are the implications of exceeding a CO2 concentration of 400 ppm?
Some climate change skeptics will be quick to highlight the scientific evidence that shows the atmospheric CO2 has been at this level and higher on the past. They are absolutely correct. Somewhere in the region of three to five million years ago similar levels were present and the world was, on average, three or four degrees Celsius warmer with some areas up to ten degrees warmer. Life survived and flourished and it is likely to have influenced the evolutionary path that led to Homo Sapiens but humanity in its current form has never experienced the like, and as far as we can tell, the changes in CO2 levels were never as rapid as over the last 150 years.
We will not wake to a radically different world tomorrow. Climate change does not work like that. If we maintain CO2 levels at 400 ppm, without any further increase, it will take decades or even centuries for a new "normal" climate to become established. Some areas will dry out, others will become wetter, some will cool and others warm, all of which will change natural vegetation and agricultural practices which will feed back into local and global weather systems. The biggest threat for the human population will be the increased frequency of crop failures. In order to cope with the still growing human population combined with climatic changes will require more wilderness to be tamed for agriculture. Perhaps the tundra will be drained for fields once the permafrost melts.
Longer term effects will be the thawing of glaciers and ice caps which will raise sea levels and affect ocean currents and hence the climate. Some estimates indicate that when the world last saw 400 ppm CO2 sea levels were 40 metres higher than now. While a rise in sea level of that magnitude may not happen for a century or more, it would be catastrophic for the world as we know it. It wouldn't just be island states such as the Maldives that would disappear but large parts of low lying countries, such as Bangladesh and the Netherlands. Our ever more crowded cities will be badly affected too. The first few floors of the newly topped out One World Trade Center in New York will be submerged as will many parts of the Eternal City, which has been inhabited for two and a half millenia. Some of the world's most densely populated cities, such as Tokyo, Cairo and London, will be much depleted.
This will lead to hundreds of millions, perhaps billions, of people being displaced by rising sea levels again putting pressure on the wilderness. The new homes and infrastructure that will be required on a scale never before experienced will demand vast quantities of energy and natural resources. It won't be the end of the human race but it may suspend our humanity as wars, famine and pestilence of biblical proportions are likely as greater numbers compete for limited resources.
Such a dark future is by no means certain. We still have a little time to act, but not long. The more we procrastinate the harder it will be to head off the worst extremes coming our way. If we continue with "business as usual" we are on target for 450ppm by the middle of the century, accompanied by six degrees Celsius temperature rises. That doesn't bear thinking about.