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 University of Colorado, adds:

Dr. Magi’s response should emphasize 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).

Todays temperature are, in fact, historically exceptional, not unexceptional as proposed in the question. The analysis by NASA indicates that global surface temperatures have increased 0.8C (1.5F) from 1880, based on the instrumental record. Going back further in time, it is clear that this warming is significantly outside the longer-term historical pattern, with the period 1000-1400 being about -0.2C (negative meaning less than the long-term average), and the year 2000 being about +0.4C from this long term mean (reference fig 16-12 of Ruddiman (reference below), taken forom Jones and Mann 2004).

There is not scientific doubt that adding CO2 to the atmosphere will cause the climate to warm. Humans have increased the atmospheric CO2 concentration from 280 to 390ppm since the pre-industrial times (~1850). The observed warming is absolutely consistent with this increase in CO2.

Ruddiman’s book Earth’s Climate: Past Present and Future (2008) is suggested as further reading.

Dr. Thomas M. Cronin writes:

I would add the following comments about the Medieval Warm Period:

Interpretation of the patterns and causes of the Medieval Warm Period, also called the Medieval Climate Anomaly, are complex and depend on the timescale over which one views the paleoclimate record. When viewed over the last 8000 years of Holocene interglacial climate, the MWP and LIA are the latest and largest of a series of millennial-scale climate anomalies reconstructed from ice cores, glacial geology, marine sediment cores, speleothems and other archives. There are reconstructions of sea level, glacier advances and retreats, Arctic sea ice, precipitation patterns, deep-ocean circulation, shifts in the intertropical convergence zone, monsoon variability, as well as atmospheric and ocean temperature. Some researchers have postulated a ~ 1500 year quasi-periodicity, others have proposed the solar variability as a possible cause, but neither of these hypotheses are conclusive. Still, Holocene interglacial climate variability including the MWP-LIA is real, although it has not been reconstructed on an annual basis, mainly decadal to centennial time resolution [see below]. This does not address the magnitude of Holocene climate variability in relation to recent warming.

Other groups have focused on late Holocene temperature viewing the MWP-LIA interval from the standpoint of recent trends, that is, in the context of recent warming. This work emphasizes the “hockey stick” shape of the mean annual northern hemisphere surface temperature (MAT) record mainly derived from proxy tree rings, plus a few ice cores, corals and sediment records and instrumental records. Most of the proxy records are annual resolution, some decadal scale. Note that the reconstructed late Holocene MAT curve now extends back about 2000 years and is mainly based on terrestrial proxy and instrumental records. Based on such reconstructions of MWP MAT warmth, and climate modeling, independent research groups confirm that MWP MAP, at least decadal averages, did not exceed recent warming and that recent warming can be explained by GHG forcing, not by solar or volcanic processes.

In addition, an increasing number of high-resolution [decadal] marine records, not yet integrated into any hemispheric or global temperature curve, are beginning to shed light on ocean temperature variability during this time. This is an important research field because it is likely that ocean-atmosphere multi-decadal processes influence the temperature variability seen in the wiggles in the late Holocene temperature curve, such as those during the 12th through 18th centuries. Also, some studies show asynchronous warming in various ocean regions during the period ~ 600-1400 CE [calendar years] adding complexity to climate processes during the MWP.

There are many references on the above topics that can be provided on request.