By Richard Black
Environment correspondent, BBC News website
Human emissions now clearly obscure solar changes as a cause of warming
One of the main reasons why governments decided two years ago to draw up a new global agreement on climate change was a major report published just before that year's UN climate summit in Bali.
Its top-line conclusions produced a collective and decisive political move in favour of a global agreement bigger and bolder than anything seen before in the environmental field.
This was the
Fourth Assessment Report (AR4)
from the Intergovernmental Panel on Climate Change (IPCC) - the distillation of research into climate past and present, melting ice, atmospheric changes, dates of bird migrations and harvests, projections of future social and economic development, in fact any field that could throw light on how the climate was changing and how it might change over the next century, and what implications that carried for humankind and the natural world.
Now, as delegations wend their way to Copenhagen for the summit that was supposed to finalise that new global agreement, one question delegates might be asking is: what have scientists discovered since the AR4 that might influence decisions?
"Overall I would say that all the new elements that were published since have only confirmed or emphasised what the IPCC wrote in its report," says Jean-Pascal van Ypersele, the organisation's vice-chairman, who is otherwise employed at the Catholic University of Louvain in Belgium.
"Actually, the state of climate knowledge has been remarkably stable over the last 20 years. Of course it's much more sophisticated, we have numbers when we only had qualifications in broad terms; but overall what we know today was already quite well known 20 years ago."
Spend enough time in the company of what you might term "mainstream" climate scientists, and you realise just how much frustration there is over the notion - widely promulgated in some circles since the AR4 - that temperatures are not rising anymore.
Their frustration stems from three main arguments:
- Of the two widely-used global temperature records, only one - the UK HadCRUT3 record - shows an apparent plateau from 1998 to 2008, whereas the other - the US GISTEMP record - indicates that temperatures have risen since then
- It is self-evident that natural cycles of cooling and warming will sometimes add to the steady warming influence of elevated greenhouse gas levels and sometimes obscure it - and that is what is happening now
- 1998 was an abnormally warm year because of the strong El Nino effect, whereas 2008 was cooled by the opposite phenomenon - La Nina - so choosing this period is "cherry-picking".
Mike Lockwood, a physicist from the UK's Reading University whose work has helped quantify the relative influences of various factors on temperatures, is especially vehement about the third of these points.
"Why pick a date other than it suits your argument?" he says.
"If I take 1997 or 1999 as the starting point, temperatures rise; so what right have you got to take 1998? That's bad science - a really specious argument."
MODERN TEMPERATURES: STABLE OR WARMING?
The HadCRUT3 record is maintained by the UK Met Office and the University of East Anglia's Climatic Research Unit, GISTEMP by Nasa's Goddard Institute for Space Studies
The datasets use much of the same raw data, but differ in how they correct for errors and allow for variations in data availability
GISTEMP uses an earlier baseline period than HadCRUT3, which is why it appears on our graph to give consistently higher temperatures
For regions where observations are missing, GISTEMP interpolates from nearby observations whereas HadCRUT3 only averages the available observations
HadCRUT3 has 1998 as the warmest year in the record, followed by 2005 and 2003; GISTEMP has 2005 as the warmest, followed by 2007 and 1998
New life was breathed into the idea of "global cooling" a few months ago when Mojib Latif, a climate modeller from the Leibniz Institute of Marine Sciences at Kiel University in Germany, presented research at a conference in Switzerland indicating that temperatures might remain stable for a further decade.
The projection was based on a climate model that tried to do something relatively new - to start from where we are now, factoring in the actual current state of natural cycles in the world around us - enabling projections that in principle would be valid over a decade or so, rather than the longer-term runs of more traditional models that were all the IPCC had to work with.
"What we did was to initialise the models, provide them with some estimate of the observed climate state," he says.
"This might sound weird because people may think that we know the current climate state; and we have good data for the atmosphere, but we also need the oceans where we don't have much data, and that's a problem."
His team factored in sea surface temperatures; a similar project by a UK Met Office team used measures of temperature and salinity.
The next decade may bring cooling - but not enough for snow on the beach
They ended up with different projections. The Kiel team projected stable temperatures for perhaps a decade, then a rapid rise; the Met Office model suggested that we will see noticeable warming within a few years.
"This is completely new science, and of course when you do new science things are uncertain," says Professor Latif.
It was surprising that his talk caused such a stir given that the scientific paper on which it was based had been published a year earlier in the journal Nature, with BBC News and other media organisations
reporting it at the time
He says that he made the limits of the research and the uncertainties plain during his presentation - but that this aspect of his talk was missed by journalists present, and has been neglected by "sceptics" since.
In any case, he says: "This has basically no implications for the long-term evolution - in the long run, global warming is not affected by this but by
greenhouse gas concentrations."
Although short-term modelling is one aspect of climate science that has definitely moved on since the AR4, the jury is still clearly out on what it is saying about climate change in the next decade.
Another area on which new light has been cast since 2007 is a possible solar influence on climatic change - in particular, how much of the warming seen in the last few decades can be laid at the door of solar changes, and whether the decline in solar output in the last few years is related to the slowdown or halt - take your pick - in temperature rise.
IPCC PROJECTIONS FOR 2100
Probable temperature rise between 1.8C and 4C
Possible temperature rise between 1.1C and 6.4C
Sea level likely to rise 28-43cm
Arctic summer sea ice disappears in second half of century
Increase in heatwaves very likely
Increase in intensity of tropical storms likely
Mike Lockwood has just published a review paper in the Royal Society's journal Proceedings A that collates the latest research.
The Sun has calmed rapidly (by its own standards) since reaching what some physicists term a "grand maximum" of output in the second half of the 20th Century - the period during which scientists have been able to observe our star from space.
"As it happens, the modern space age has seen an unusually high level of solar activity," he says.
"But now we're back to a level last seen in about 1924."
If the Sun was the dominant influence on modern-day temperature change, as some assert, then why has the Earth not returned to temperatures of the 1920s - which would have entailed a cooling of about 0.6C over the last couple of decades?
One explanation could be that there is a long timelag between solar output changes and their effect on terrestrial temperatures.
But, says Professor Lockwood: "To get back to 1920 levels of solar activity and still be above 1980 levels in temperature would require a time constant (lag) of more than 40/50 years.
"But we know that's not the case, because we can see a small signal of the 11-year solar cycle in tropospheric temperatures, and if we had a 40/50 year lag that would be smoothed."
Another reason for banishing the solar notion is that changes in solar output are far too small by themselves to cause the kind of temperature changes seen on Earth.
They have to be amplified by some mechanism; and although we do not understand the mechanism, observations tell us the degree of amplification - it is about three.
By comparison, Professor Lockwood outlines, something approaching 30 would be needed to explain the observed temperature fluctuations.
Although the solar effect is real, he says, the notion that it is the dominant driver of modern-day climate change is even more demonstrably wrong than when the IPCC was compiling its 2007 report.
"People's capacity for self-delusion is extraordinary," he says.
"I'd love it to be true; but wanting something doesn't make any difference to the science. You cannot spin away a scientific reality."
Clouding the picture
One of the AR4's uncomfortable realities was an inability to make projections of sea level rise that were both comprehensive and meaningful.
In the years leading up to the report there had been indications that glaciers in Greenland and parts of Antarctica were speeding up, dropping ice into the oceans at a faster pace, which might translate into faster sea level rise.
However, the IPCC was forced to acknowledge that it could not produce a meaningful estimate of how much this might raise the waters, because understanding of the physical processes involved was little more than embryonic.
So whereas the IPCC put the likely upper limit of sea level rise at 43cm by 2100, it also said this was an underestimate.
The intervening years have brought more observations, both on the ground and from space. Satellite missions can measure changes in mass of the polar ice sheets, changes in ice height and the speed of glaciers.
The contribution of clouds has yet to be quantified to everyone's satisfaction
The picture all this paints
- some glaciers slowing down again on Greenland and others accelerating, more rapid mass changes than expected in West Antarctica, and
hints of mass loss from East Antarctica
Academics have responded with projections that range up to about one and a half metres.
So if the IPCC were producing its assessment now, what sort of projection might it make for sea level rise in 2100?
"I'd bet less than a metre," says Simon Holgate from the UK's Proudman Oceanographic Laboratory, home of the Permanent Service for Mean Sea Level, the global observation network.
"I think the IPCC might be a bit on the low side, but I think work that's happened since then suggests that some of the fears about fast ice dynamics are unfounded."
He also notes that the observed rate of sea level rise - now determined from satellites as well as tide gauges - is hardly changing.
"It seems to be linear [at about 3mm per year], which is interesting in a number of ways.
"The satellite record doesn't show signs of variability as there are in the tide gauge records, or signs of acceleration."
The IPCC would also like to narrow the range of its projections for temperature rise - 1.8-4.0C from pre-industrial times by 2100 - and
the main scientific uncertainty involved
is the extent to which clouds amplify or ameliorate temperature changes.
Here, there has been little progress, says Professor van Ypersele.
"In the area of clouds, it's very frustrating," he says. "Progress in cloud representation has been quite slow."
So although some aspects of the scientific picture are clearer now than they were in 2007, others are not.
The broad outline, though, deviates little from the IPCC's conclusions -unequivocal evidence of warming, more than 90% likelihood that humanity's emissions of greenhouse gases were principally to blame, projections of temperature and sea level rise, declining crop yields, mountain glacier melt, and considerable damage to ecosystems and the human economy.