Oftentimes, World Climate Report focuses on how elevated atmospheric levels of CO2 benefits various organisms or how observed changes in elements of climate in specific regions are not consistent with expectations from numerical climate model experiments. We could almost feature an article on climate change and hurricanes every week—these kinds of articles are found throughout the peer-reviewed scientific journals. But we don’t want to lose sight of the big picture—the term “global warming” implies that the world is indeed warming, humans are somehow responsible, and we better change our evil ways or we could inadvertently destroy of climate system. So here, we’ll review a couple of “Big Picture” articles from the recent scientific literature to see if things really are as cut and dry as they are implied.
The first “Big Picture” article was written by Fredrik Ljungqvist of the University of Sweden and was published in Geografiska Annaler, which is associated with the Swedish Society from Anthropology and Geography. The author was interested in reconstructing the temperature of the extra-tropical Northern Hemisphere for the past two millennia, and to do so, he search around for sensitive proxy records spread relatively evenly throughout the hemisphere. Ljungqvist reports “The new reconstruction presented in this paper consists of 30 temperature sensitive proxy records from the extra-tropical Northern Hemisphere (90–30°N), all of which reach back to at least AD 1000 and 16 all the way back to AD 1.”
As seen in Figure 1, the author achieved the goal of locating proxy records from many parts of the Northern Hemisphere. Regarding the types of records, he states “A wide range of different kinds of proxies with annual to multi-decadal resolution have been used” including “2 historical documentary records, 3 marine sediment records, 5 lake sediment records, 3 speleothem δ18O records, 2 ice-core δ18O records, 4 varved thickness sediment records, 5 tree-ring width records, 5 tree-ring maximum latewood density records, and 1 δ13C tree-ring record.” We note that these are all commonly used proxies used in climate reconstruction research—in fact, we have covered most of them many times in our essay series. Ljungqvist comments “Virtually all available high-quality palaeotemperature proxies with a reasonably high temporal resolution have been used.”
Figure 1. The geographical locations of the proxy records (from Ljungqvist, 2010).
If you examine Figure 1, you may question why so few records come from the United States, particularly the southwestern US. The author explains “However, all tree-ring width records from arid and semi-arid regions, as southwest USA and Mongolia, have been excluded from the reconstruction. Since they may have been affected by drought stress, they possibly do not show a linear response to warming if higher summer temperatures also reduce the availability of water.” Fair enough.
Figure 2 is the bottom line of this research. The author concludes:
This reconstruction is the first to show a distinct Roman Warm Period c. AD 1–300, reaching up to the 1961–1990 mean temperature level, followed by the Dark Age Cold Period c. AD 300–800. The Medieval Warm Period is seen c. AD 800–1300 and the Little Ice Age is clearly visible c. AD 1300–1900, followed by a rapid temperature increase in the twentieth century. The highest average temperatures in the reconstruction are encountered in the mid to late tenth century and the lowest in the late seventeenth century. Decadal mean temperatures seem to have reached or exceeded the 1961–1990 mean temperature level during substantial parts of the Roman Warm Period and the Medieval Warm Period. The temperature of the last two decades, however, is possibly higher than during any previous time in the past two millennia, although this is only seen in the instrumental temperature data and not in the multi-proxy reconstruction itself.
Figure 2. Estimations of extra-tropical Northern Hemisphere (90–30°N) decadal mean temperature variations (dark grey line) AD 1–1999 relative to the 1961–1990 mean instrumental temperature from the variance adjusted 90–30°N thermometer record (black dotted line showing decadal mean values AD 1850–1999) with 2 standard deviation error bars (light grey shading) (from Ljungqvist, 2010).
We note that the reconstruction ends with 1999, and while the extratropical Northern Hemisphere has warmed a bit since then according to surface thermometers, still the nature of the temperature variability over the past two millennia as captured by the proxies does not seem to suggest that current times are so beyond anything else to be particularly noteworthy. Yes, times are warm, but clearly the Northern Hemisphere extra-tropical average temperature moves around a lot, and so current conditions should not come as much of a shock to natural systems, which are accustom to high levels of natural variability.
Fredrik Ljungqvist continues to build upon this work, and his latest results have been submitted to Climate of the Past which is an open access journal which invites discussion. To see Ljungqvist’s new results, or the discussion taking place about them, visit http://www.clim-past-discuss.net/7/3991/2011/cpd-7-3991-2011-discussion.html.
This brings us to the second article entitled “Recent energy balance of Earth” and published in the International Journal of Geosciences. The title certainly suggests a “big picture” look at the global warming debate.
Knox and Douglass explain that since the early 2000s, autonomous Argo floats have been deployed throughout the world providing highly accurate measurements of the heat content of the ocean (Figure 3). There are over 3,000 floats operating at any one time, and each measures heat and salinity to a depth of 2,000 meters.
Figure 3. Distribution of 3,000+ Argo floats.
The measurements are shown in the Figure 4, and incredibly, the Argo floats have detected cooling, not warming. The authors explain “Using only 2003- 2008 data, we find cooling, not warming. This result does not support the existence of a large frequently-cited positive computed radiative imbalance.” Furthermore, they note “In summary, we find that estimates of the recent (2003-2008) OHC rates of change are preponderantly negative.”
Figure 4. Ocean heat content from Argo (left scale: blue, original data; red, filtered) and ocean surface temperatures (right scale, green). Conversion of the OHC slope to W/m2 is made by multiplying by 0.62, yielding –0.161 W/m2 (from Knox and Douglass, 2010)
We fully understand that a period from 2003 to 2008 is far too short to make any grandiose proclamations about the state of warming of the planet. And in fact, several updates to this data set (and other similar data sets), indicate that a very slight warming trend is now present since 2003—although the magnitude of this warming is still a considerable amount less than expected.
Combine a variable natural climate with sluggish warming in the recent years and you find that the anthropogenic part of global warming may not always be the dominant part. For these and the other reasons that we often feature at World Climate Report, the “Big Picture” is not anywhere near as clear as you are often led to believe.
Knox, R.S., and D.H. Douglass. 2010. Recent energy balance of Earth. International Journal of Geosciences, 1, 99-101.
Ljungqvist, F.C., 2010. A new reconstruction of temperature variability in the extra-tropical Northern Hemisphere during the last two millennia. Geografiska Annaler, Series A, 92, 339–351.
Christiansem, B., and F. C. Ljungqvist, 2011. The extra-tropical NH temperature in the
last two millennia: reconstructions of low-frequency variability. Climate of the Past Discussion, doi:10.5194/cpd-7-3991-2011, http://www.clim-past-discuss.net/7/3991/2011/cpd-7-3991-2011.pdf