Background

The surface of the Earth is heating up. An increasing number of climatologists, approaching a scientific consensus, recognize and acknowledge that the evidence for warming is beginning to exceed the uncertainty in our ability to measure such changes.  Anecdotal indications are also flooding into the public awareness: record-high temperatures, retreating mountain glaciers, thawing permafrost, disappearing polar sea ice, frequent hurricanes, the list goes on and on.  Yet the issue of attribution, whether human activity is playing a role in the observed changes, remains contentious.  Public perceptions are influenced alternately by predictions of disaster in popular media and disavowals by concerned economic interests. Opinionated pundits line up on each side of the issue, and the discussions are increasingly politicized.  Yet there is a large and growing body of rationale, rigorous scientific investigation on a number of fronts that has a substantial potential to inform the public debate.

One line of study includes the detailed observation of modern changes.  Examples include addressing the questions of whether the Greenland ice sheet is shrinking or growing, or whether comparable warming affects both the lower and upper atmosphere.  Although these are crucial questions, in isolation they may only address what is happening without shedding light on why.  An equally important aspect of the problem is the consideration of natural climate variability and progression during a warm interval, above which human influences may be recognized.  Because human influences extend to almost every corner of the Earth, natural variability is best investigated by studying the past, through the field of paleoclimatology.  The broader public may be less aware of the scientific progress and remaining issues of paleoclimate, so the proposed Morss Colloquium will provide the dual benefit of bringing together a group of experts in various aspects of global climate change and also introducing the scientific knowledge and debate to the wider public audience.

The development of most aspects of human civilization has taken place within the last 10,000 years.  Agriculture, organized societies, technological advances have all coalesced during this interval, known as the Holocene.  It has been a time of relatively warm and in some ways remarkably stable climate.  The preceding ice age, indeed at least the previous 100,000 years, were marked by dramatic climate instability, with repeated abrupt climate oscillations larger than anything experienced by human society.  The Holocene has also been called the post-glacial period, but this is most likely a geological misnomer, implying that there was an ice age and that it is over. The truth is that the Earth has generally been in a glaciated mode for the past several million years, with increasingly severe ice ages during the last million years.  These ice ages are some of the most extreme conditions since at least before the age of the dinosaurs, hundreds of millions of years ago.  The recent ice ages have been interspersed with geologically brief interglacial periods such as the Holocene when milder conditions prevailed.

The study of past natural climate variability that is most relevant to modern assessments is therefore ideally focused on warm interglacial intervals.  One obvious candidate is the pre-industrial Holocene. Yet this most recent interglacial interval, in which we live, has not run its natural course, and the question of whether and when it would end, and what the natural climatic progression might be, cannot be addressed by looking solely at the elapsed portion of several thousand years since the Holocene began.  Previous completed interglacial intervals hold much more promise for presenting the entire picture of natural climate variability during a warm time.Because the alternating glacial and interglacial intervals are easily recognized in oxygen isotope records from the deep sea, previous climates are commonly referred to by using the marine isotope stages (MIS), counting back in time from today (MIS 1) with warm intervals assigned odd numbers and ice ages assigned even numbers.  The last time the Earth was as warm as the pre-industrial time was during MIS 5, although significant differences in climate forcing make this an appropriate but not ideal candidate for study in order to provide insights into the present and future.  A better candidate, MIS 11, occurred approximately 400,000 years ago following a glaciation that rivaled the last ice age in severity.

During MIS 11 the climatic influences on the Earth were most similar to those that would characterize the Holocene and future in the absence of human activity.  Simply put, these climate influences are the amount and seasonal distribution of sunlight, and the magnitude of trapping of outgoing radiation by so-called greenhouse gases in the atmosphere.  Variations in solar output, such as accompany sunspot cycles, are extremely small, but much larger changes in the timing and location of sunlight occur because of changes in the Earth’s orbit.  These changes, most famously calculated by a Serbian mathematician (Milankovitch, 1941) have been shown to be most similar between today and 400,000 years ago (Loutre and Berger, 2000).  The shortwave radiation of the sun that is incident on the surface of the Earth is emitted back as longer wave radiation, some of which is absorbed by greenhouse gases such as water vapor, carbon dioxide, and methane, warming the atmosphere.  Recent measurements of ancient air trapped in bubbles within Antarctic ice indicate that greenhouse gases varied systematically over previous glacial cycles, returning repeatedly to pre-industrial values during the last few interglacial intervals, beginning with MIS 11.  So the intervals studied before MIS 11 did not achieve the same greenhouse gas trapping, and the intervals since then were characterized by larger insolation cycles than either during MIS 11 or the Holocene.  These similarities in climate influences make MIS 11 perhaps the ideal analogue for natural climate variability for comparison to today and the future.