How does rising atmospheric CO2 affect marine organisms?

Click to locate material archived on our website by topic

The Little Ice Age and Medieval Warm Period: Were They Real?
Grove, J.M.  2001.  The initiation of the "Little Ice Age" in regions round the North Atlantic.  Climatic Change 48: 53-82.

What was done
The author reviews the relevant scientific literature in an attempt to determine the time of initiation of the Little Ice Age in the regions surrounding the North Atlantic Ocean, focusing specifically on the Swiss Alps, Canada, Greenland, Iceland, Spitsbergen, and Scandinavia.  The nature of both the Little Ice Age and Medieval Warm Period are also discussed.

What was learned
With respect to the latter of these two subjects, Grove notes that the nature - and even the existence - of both the Medieval Warm Period and Little Ice Age have been questioned in recent years.  In response to this revisionist tendency, she acknowledges that these periods were not characterized by unbroken warmth and cold.  Thus, she defines the Little Ice Age as "the most recent period during which glaciers extended globally and remained enlarged, while their fronts fluctuated about more forward positions," and she defines the Medieval Warm Period as "the interval between the 'Little Ice Age' and earlier general expansion episodes" of globally-occurring glaciers.

In harmony with these glacier-behavior-based definitions, Grove goes on to say that "climatic conditions were such during the 'Little Ice Age' that mass balances were sufficiently predominant for the glaciers to remain enlarged, although their fronts oscillated."  Similarly, she says that "during the 'Medieval Warm Period' climatic conditions caused mass balances to be negative, and volumes of glaciers to be reduced, so that they retracted substantially, though their fronts no doubt oscillated, as they have been observed to do during the warming of the twentieth century."

Last of all, and in what is really the main concern of this specific paper, Grove concludes from her study of the literature that "the 'Little Ice Age' was initiated before the early fourteenth century in regions surrounding the North Atlantic."  And in a second reference to the worldwide nature of this climatic phenomenon, she declares that "field evidence clearly shows that glaciers on all continents expanded and fluctuated about forward positions during recent centuries."

What it means
This meticulous and insightful scholar - who in the words of Ogilvie and Jonsson (2001) has written "the seminal work [Grove, 1988] on the topic" - thus confirms what we have long been contending on our website, i.e., that the Little Ice Age and earlier Medieval Warm Period were both real (in that they actually occurred) and global (in that they occurred on every continent).  Viewed in this light, it is clear that the modest warming of the 20th century is likely nothing more than an expected phase of the natural millennial-scale climatic oscillation that alternately shepherds the planet back and forth between these two moderate extremes of normal Holocene climate (see the many Journal Reviews and Editorials dealing with millennial-scale climate oscillations listed under Climate Oscillations in our Subject Index).

Climatologically speaking, therefore, nothing unusual is happening; the earth is merely returning to a climatic regime much like that of the Medieval Warm Period, which we could perhaps call the Modern Warm Period.  Mankind is not responsible for this transition; but we will surely benefit from it, as we will also benefit from the many positive effects of the concurrent and ongoing rise in the air's CO2 content on plant growth and development, for which we likely are responsible ... and thankfully so, as Cowling (1999) has rightly reminded us (see our Editorial of 15 October 1999).

Cowling, S.A.  1999.  Plants and temperature - CO2 uncoupling.  Science 285: 1500-1501.

Grove, J.M.  1988.  The Little Ice Age.  Methuen, London, UK.

Ogilvie, A.E.J. and Jonsson, T.  2001.  "Little Ice Age" research: A perspective from Iceland.  Climatic Change 48: 9-52.