Got a question about climate science? These next few weeks at Rooted we’re running the Ask a Climate Scientist series. Keep the questions coming as well, by either leaving one as a comment, or emailing directly to me. These answers are coming from American Geophysical Union’s Climate Science Q&A service, where more than 700 volunteer scientists provide factual and peer-reviewed climate science information to journalists. The AGU only comments on science, not climate policy.

These three questions come from well-known Crikey climate sceptic commenter Tamas Calderwood.

Question 1: According to satellite measurements, the world hasn’t warmed since 1998 despite record human CO2 production. Why?

Brian I. Magi, from Princeton University, writes:

1998 was a year with a very strong El Nino, which is thought to have helped produce the strong positive temperature anomaly. If 2009 is compared to 1998, then the world has not warmed (much).  However, if 2009 is compared to 1997 or 1999, for example, the world has warmed.  The satellite-based temperature records are available for UAH and RSS, and for the weather-station based temperature record from NASA GISS. Other temperature record sources are available too.

The best thing to do, if you have doubts, is plot those numbers in a figure (using Excel, for example) and see for yourself. Further discussion with respect to the statistics of trends in a time series like temperature can be found at various blogs like this one.

Kevin Schaefer, from the National Snow and Ice Data Center at the Univserit of Colorado, adds:

Dr. Magi’s response should emphasise that only trends from the full temperature record are important (not just after 1997). A single anomalously warm year, like 1998, does not significantly affect the overall 1978-present trend.

Question 2: Why were the warming spurts from 1860-1880, 1910-1940 and 1975-1998 all of the same magnitude, given that CO2 concentrations were higher in the later periods?

Dr Kelly Halimeda Kilbourne, from the University of Maryland, writes:

The climate system has many frequencies of variability. Everyone is familiar with daily cycles and annual cycles, but there are other frequencies too. Global temperature has variations that occur over decades and multiple decades. The causes of that variability is currently a very active research topic and there are different competing ideas right now about the causes.

The important part with regards to global warming is that decadal-scale variability is superimposed on the gradual temperature increase due to human inputs of greenhouse gases. Thus, sometimes the decadal-scale variability makes global temperatures rise faster than we would expect due to greenhouse gas emissions and sometimes the decadal-scale variability temporarily masks the warming due to greenhouse gas emissions.  The time periods you point out were periods when decadal variability added to the global warming signal and made the temperatures rise more than one would expect just due to greenhouse gases.

Galen A. McKinley, from the University of Wisconsin, reviewed Dr Kilbourne’s answer and found it appropriate.

Question 3: What caused the medieval warm period, and why do you rule out natural variability for long term warming given that today’s temperatures are historically unexceptional?

Galen A. McKinley, from the University of Wisconsin, responded:

Evidence for the Medieval warm period is not as strong as for the Little Ice Age in the proxy records. Nevertheless, such a warm period from about 1000-1400 is not inconsistent with other trends occurring in the high latitudes of the northern hemisphere.

In this period, the Arctic received more solar radiation in summer than presently, which would have contributed to the melting of ice and more solar shortwave being absorbed (“ice-albedo feedback”). Orbital changes have slowly reduced the amount of incoming solar radiation in the Arctic over the past 6000 years have helped to cool the northern hemisphere.

Other possibilities are that (1) ocean circulation may have caused increased northward heat transport during this period; (2) solar variability may have made a small contribution; and (3) volcanic eruptions may have also contributed (but records for this are sparse).