Is the United Nations goal of limiting global warming now out of reach? 2°C warming is the globally agreed target set to avoid dangerous climate change reiterated at the 2009 UN Copenhagen Accord. This is now nearly impossible according to a recent study by researchers from British Columbia, Canada  (see also this paper ).
“The paper finds that reaching that goal would require that greenhouse emissions “ramp down to zero immediately” and that scientists deploy means, starting in 2050, to actively remove greenhouse gases from the atmosphere. Previous modeling (, ) efforts have already highlighted the difficulty of reaching the 2˚C goal.” Science Now
 V. K. Arora et al., Carbon emission limits required to satisfy future representative concentration pathways of greenhouse gases, Geophys. Res. Lett., 38, L05805, doi:10.1029/2010GL046270, 2011.
 K. Anderson and A. Bows, Beyond ‘dangerous’ climate change: emission scenarios for a new world, Phil. Trans. R. Soc. A, 369, 20-44, doi:10.1098/rata.2010.0290, 2011.
 M. Meinshausen et al., Greenhouse-gas emission targets for limiting global warming to 2°C, Nature, 458, doi:10.1038/nature08017, 2009.
 M. R. Allen et al., Warming caused by cumulative carbon emissions towards the trillionth tonne, Nature, 458, doi:10.1038/nature08019, 2009.
Some suggest a global 2 degree Celsius limit is rather arbitrary and policy makers should instead focus on regional impacts and legislate to prevent so-called tipping points. See this opinion piece by Tim Lenton in Nature. This idea is a good future aspiration, although not sensible for present as we do not know well enough regional impacts or when potential tipping point might be reached; furthermore, a global rise in temperature of two degree Celsius will almost certainly already mean some serious regional impacts for some parts of the planet. Tim does suggest that the United Nations Framework Convention on Climate Change (UNFCCC) be extended to limit the rate and gradients of climate change and this indeed could be a helpful starter.
Wally Broecker, an outstanding American scientist and pioneer of Earth System Science, in a book (fixing climate) published in 2009 suggested, somewhat controversy, that we implement large-scale carbon dioxide removal from the atmosphere. I like his reasoning – in the past we dumped our sewage in the streets, until we learnt about the spread of disease and the importance of hygiene, so the Victorians built a sewage network to dispose of the waste. Likewise, we are dumping carbon dioxide, a waste byproduct of burning fossil fuels, into the atmosphere. We now realise that this is leading to climate change with potential devastating consequences. We therefore need to employ an extensive network of machines to scrub carbon from the atmosphere and pump it into deep geologic permanent storage. However, the trouble is that this technology has not been tested at industrial scale, will cost a fortunate, will need massive widespread implementation, and the feasibility and capacity of storage sites is dubious.
The bottom line – we must aggressively cut all our greenhouse gas emissions and the sooner we start the better chance we have of limiting dangerous climate change.