The thing that makes science unique in all human endeavour is that it is independently verifiable. In this regard, the Berkeley Earth Project (composed largely of physicists outside of the climate community) recently announced their independent assessment of land surface temperature trends. This study was motivated after the impartiality of groups like the University of East Anglia Climate Research Unit and the NASA Goddard Institute for Space Studies was questioned. The Berkeley Earth Project assessment of trends is shown in the figure, below. In fact, the Berkeley numbers are remarkably similar to the NASA GISS estimates, and suggest greater warming than CRU were claiming. In retrospect (and indeed at the time), the idea that such a large number of climate scientists in full view of an even wider audience of geophysicists and planetary scientists could have colluded in a multi-decade-long sham reflects a profound ignorance of how science itself functions amongst a distressing large fraction of the populous. That said, independent checks are an essential part of science, and it’s fantastic that this was done.
On a side note, sometimes the problem of climate change is inverted in public discussion from how I typically conceive it (and opposite to the direction of the underlying uncertainty). The question shouldn’t be “Why is the Earth warming?” (since it’s only relatively recently that we’ve gained confidence that it is). Rather, it has always been much more compelling and reasonable to ask: “Given that we know we’re dumping CO2 into the atmosphere to concentrations not seen in the last million years – and we know that CO2 is a greenhouse gas – what is the likely impact on climate going to be?” I’ve attached two figures that show the recent trends in CO2 and the record over the last 500,000 years. There is no credible argument that the CO2 rise is due to anything but fossil fuel burning and human land-use changes. The expected impact of these kinds of CO2 increases are also consistent with (but there is a broad band of uncertainty here) the types of temperature changes observed. At this point, plausibility is very strongly on the side of our causing the observed temperature increases with CO2 emissions (with a little CH4 and water vapour feedback for good measure).
From a political point of view, it’s probably more appropriate now to be debating what kind of response is optimal – is it cheaper to try and reduce CO2 and CH4 emissions, or is it better to “book keep” for cost impacts associated with a warming climate and just let it evolve? Likely a mix of those is most credible. In any case, that seems to be the message that’s becoming clearer as each year goes by – the questions of if we’re warming and why are resolved (a definitive “yes” and “human greenhouse gas emission”). By how much (and more importantly, with what impact) and what we should do about it or in response to it remain open.