The Berkeley Earth project (link), a scientific organization founded by global warming skeptics and devoted to independent analysis of global temperature data, agrees with the Japan Meteorological Agency and with NASA and NOAA in the U.S. that the year 2014 was likely the warmest since reliable records began in 1850.
Their report on 2014 (link to PDF) is careful to note that taking into account the statistical margin of error, it is not completely certain that 2014 was warmer that the next warmest years, 2005 and 2010. The overall pattern still seems clear, however.
(As astronomer Phil Plait noted in a blog post I mentioned yesterday [link], NASA's calculations also show a large enough margin of error to allow for the possibility that 2005 or 2010 was as warm or warmer, though NOAA's results, computed slightly differently, show 2014 as definitely the hottest.)
The Berkeley Earth Surface Temperature (BEST) project, as Berkeley Earth was originally known, was founded in early 2010 by Richard A. Muller, a physicist, and his daughter Elizabeth Muller, a consultant on energy policy and managing partner of Global Shale, a company involved in shale gas exploration and hydraulic fracturing ("fracking"). At the time, Richard Muller agreed with other global warming skeptics (few of them scientists) that the historical global temperature series assembled by climatologists weren't entirely reliable owing to such potential sources of error as the supposed urban heat island effect. Hence the BEST team set about creating a massive, independent global temperature database intended to correct what they perceived as possible or probable flaws in previous research.
The project was funded with grants from a variety of sources, with the most controversial one being the Charles G. Koch Foundation. Charles Koch and his brother David, whose large fortunes are derived from fossil fuels, are widely known for disputing the reality of global warming and also for making huge political donations in support of conservative candidates. However, according to Muller, the foundation's grant came with no restrictions or interference.
In 2011, when BEST had not yet published conclusions, Muller assured The Guardian newspaper (link), that "We are bringing the spirit of science back to a subject that has become too argumentative and too contentious. We are an independent, non-political, non-partisan group. We will gather the data, do the analysis, present the results and make all of it available. There will be no spin, whatever we find." And this appears to be exactly what they did.
In June 2012, Muller wrote an op-ed for The New York Times (link) that began,
Call me a converted skeptic. Three years ago I identified problems in previous climate studies that, in my mind, threw doubt on the very existence of global warming. Last year, following an intensive research effort involving a dozen scientists, I concluded that global warming was real and that the prior estimates of the rate of warming were correct. I’m now going a step further: Humans are almost entirely the cause.
My total turnaround, in such a short time, is the result of careful and objective analysis by the Berkeley Earth Surface Temperature project, which I founded with my daughter Elizabeth. Our results show that the average temperature of the earth’s land has risen by two and a half degrees Fahrenheit over the past 250 years, including an increase of one and a half degrees over the most recent 50 years. Moreover, it appears likely that essentially all of this increase results from the human emission of greenhouse gases.
These findings are stronger than those of the Intergovernmental Panel on Climate Change, the United Nations group that defines the scientific and diplomatic consensus on global warming. In its 2007 report, the I.P.C.C. concluded only that most of the warming of the prior 50 years could be attributed to humans. It was possible, according to the I.P.C.C. consensus statement, that the warming before 1956 could be because of changes in solar activity, and that even a substantial part of the more recent warming could be natural.
Our Berkeley Earth approach used sophisticated statistical methods developed largely by our lead scientist, Robert Rohde, which allowed us to determine earth land temperature much further back in time. We carefully studied issues raised by skeptics: biases from urban heating (we duplicated our results using rural data alone), from data selection (prior groups selected fewer than 20 percent of the available temperature stations; we used virtually 100 percent), from poor station quality (we separately analyzed good stations and poor ones) and from human intervention and data adjustment (our work is completely automated and hands-off). In our papers we demonstrate that none of these potentially troublesome effects unduly biased our conclusions.
It should be noted that Muller still holds views somewhat outside the scientific mainstream on some aspects of global warming and the consequences of the resulting climate change. But skepticism is an important part of science, and when skeptics independently investigate a phenomenon and replicate earlier results, that makes those results that much more convincing.
Update: For a short video from an interview with Muller describing the work and the conclusions, see this more recent post.