A - Do you agree that according to the global temperature record used by the IPCC,
the rates of global warming from 1860-1880, 1910-1940 and 1975-1998 were identical?
An initial point to make is that in the responses to these questions I've assumed
that when you talk about the global temperature record, you mean the record that
combines the estimates from land regions with those from the marine regions of the
world. CRU produces the land component, with the Met Office Hadley Centre producing
the marine component.
Temperature data for the period 1860-1880 are more uncertain, because of sparser
coverage, than for later periods in the 20th Century. The 1860-1880 period is also
only 21 years in length. As for the two periods 1910-40 and 1975-1998 the warming
rates are not statistically significantly different (see numbers below).
I have also included the trend over the period 1975 to 2009, which has a very similar
trend to the period 1975-1998.
So, in answer to the question, the warming rates for all 4 periods are similar and
not statistically significantly different from each other.
B - Do you agree that from 1995 to the present there has been no statistically-significant
Yes, but only just. I also calculated the trend for the period 1995 to 2009. This
trend (0.12C per decade) is positive, but not significant at the 95% significance
level. The positive trend is quite close to the significance level. Achieving statistical
significance in scientific terms is much more likely for longer periods, and much
less likely for shorter periods.
C - Do you agree that from January 2002 to the present there has been statistically
significant global cooling?
No. This period is even shorter than 1995-2009. The trend this time is negative (-0.12C
per decade), but this trend is not statistically significant.
G - There is a debate over whether the Medieval Warm Period (MWP) was global or
not. If it were to be conclusively shown that it was a global phenomenon, would you
accept that this would undermine the premise that mean surface atmospheric temperatures
during the latter part of the 20th Century were unprecedented?
There is much debate over whether the Medieval Warm Period was global in extent or
not. The MWP is most clearly expressed in parts of North America, the North Atlantic
and Europe and parts of Asia. For it to be global in extent the MWP would need to
be seen clearly in more records from the tropical regions and the Southern Hemisphere.
There are very few palaeoclimatic records for these latter two regions.
Of course, if the MWP was shown to be global in extent and as warm or warmer than
today (based on an equivalent coverage over the NH and SH) then obviously the late-20th
century warmth would not be unprecedented. On the other hand, if the MWP was global,
but was less warm that today, then current warmth would be unprecedented.
We know from the instrumental temperature record that the two hemispheres do not
always follow one another. We cannot, therefore, make the assumption that temperatures
in the global average will be similar to those in the northern hemisphere.
H - If you agree that there were similar periods of warming since 1850 to the current
period, and that the MWP is under debate, what factors convince you that recent warming
has been largely man-made?
The fact that we can't explain the warming from the 1950s by solar and volcanic forcing
- see my answer to your question D.
N - When scientists say "the debate on climate change is over", what exactly do they
mean - and what don't they mean?
It would be supposition on my behalf to know whether all scientists who say the debate
is over are saying that for the same reason. I don't believe the vast majority of
climate scientists think this. This is not my view. There is still much that needs
to be undertaken to reduce uncertainties, not just for the future, but for the instrumental
(and especially the palaeoclimatic) past as well.