Are Long-Term Climate Models Trustworthy?

Everything from global warming policy to evolutionary history depends on long-term climate models.  Textbooks make it seem like earth keeps reliable recordings that allow scientists to simply read off the record of years, decades, centuries, millennia and millions of years objectively.  It’s not that simple, wrote Maureen E. Raymo and Peter Huybers in Nature last month.¹  There’s an embarrassing amount of interpretation required.
    “Much progress has been made towards understanding what caused the waxing and the waning of the great ice sheets,” they began optimistically, “but a complete theory of the ice ages is still elusive.”  They recounted the history of theorizing about earth history, from Agassiz to Milankovitch and beyond.  Milankovitch, for instance, presented evidence that periodic orbital variations coincided with ice ages.  Scientists became convinced that earth history was decipherable with “proxy climate records” such varying ratios of oxygen isotopes, thickness of ice layers and sediments, and others.
    One serious question that arises, though, is how far back it is legitimate to extrapolate cycles that are only observable within short periods:

The climate physics and chemistry that are best understood are mainly attuned to processes that occur at daily to interannual timescales.  Are the important factors that regulate climate over centennial and longer timescales known?  When a climate model is stepped forward, by minutes or days at a time, for hundreds or thousands of years, are the final results realistic?  Climate scientists still do not understand how the subtle shifts in insolation at the top of the atmosphere are converted into massive changes in the ice volume on the ground.

It seems reckless, therefore, to speak with confidence about patterns covering vast epochs of time.  Even assuming the long-term cycles carry valid information, the trouble is, the clocks don’t always agree.  Getting the clocks to tick together is an exercise in frustration.  Here’s an example:

To tackle these problems, some researchers have turned to a time when glaciation seems to have been relatively straightforward.  The glacial cycles of the late Pliocene to early Pleistocene (~1�3 million years ago) were more regular than those of the late Pleistocene, typically lasting about 41,000 years (Fig. 1a), which matches the period of change in Earth’s tilt (Fig. 1b).  But how is the lack of variability with respect to precession explained?  Precession, which occurs mainly at 23,000-year and 19,000-year intervals, is the orbital component that most influences summer insolation intensity (Fig. 1c).  Indeed, precession is clearly observed in the ice-volume and sea-level records for the past 700,000 years.  The few computer models that have been used to study the climate history of the late Pliocene to early Pleistocene also show a strong precession signal in the modelled ice volume.  Are these climate models missing a fundamental piece of climate or ice-sheet physics, or are the assumptions about ice-volume proxies, such as delta-18 O[xygen], flawed?  Both are possibilities.

The authors discussed several of the modern approaches to tweak parameters and data sets to arrive at some measure of conformity.  The end result leaves a question whether the model is being massaged to fit the data, or vice versa.  What are they really measuring?  What do they really know?  Their final paragraph leaves more questions than answers.

…. It could be that the East and West Antarctic ice sheets have had a far more dynamic history than has been thought.
    It is widely accepted that variations in Earth’s orbit affect glaciation, but a better and more detailed understanding of this process is needed.  How can the 41,000-year glacial cycles of the early Pleistocene be explained, let alone the ~100,000-year glacial cycles of the late Pleistocene?  How do the subtle changes in insolation relate to the massive changes in climate known as glacial cycles?  And what are proxy climate records actually measuring?  The field now faces these important questions, which are made all the more pressing as the fate of Earth’s climate is inexorably tied to the vestige of Northern Hemisphere glaciation that sits atop Greenland, and to its uncertain counterpart to the south.

Similar quandaries about earth science were expressed by Caltech geologist David J. Stevenson in a supplemental article in the same issue of Nature.²  A search on the number of times he uses the word “However” is illuminating.  Some examples:

• The remarkable growth in the study and understanding of Earth has happened in parallel with a spectacular era of planetary exploration, relevant astronomical discoveries and computational and theoretical advances, all of which help us to place Earth and its interior in a perspective that integrates the Earth sciences with extraterrestrial studies and basic sciences such as condensed-matter physics.  However, progress on the biggest challenges in understanding the deep Earth continues to rely mainly on looking down rather than looking up.
• Almost a billion bodies 10 km in diameter would be needed to make an Earth.  However, it is not thought likely that planetesimals were the actual building blocks of Earth.  A dense swarm of such bodies in nearly circular low-inclination orbits is gravitationally unstable on a short timescale.
• The picture [of the origin of the moon by an impact, followed by transfer of material between Earth and the disk] was originally motivated by a desire to understand the remarkable similarity of Earth and Moon oxygen isotopes but also finds support in tungsten and possibly silicon isotopic evidence.  However, we do not yet have a fully integrated model of lunar formation that is dynamically satisfactory as well as chemically acceptable.
• We understand why Earth’s mantle convects: there is no alternative mechanism for eliminating heat.  However, we do not understand why Earth has plate tectonics.

In a third paper in the Nature supplement on geology,³ Phillip A. Allen (Imperial College, London) made the following comment: “We are right to be suspicious of oversimplistic interpretation of the ‘structure’ found in the large number of time-series records that geology throws up,” he said.  The possible allusion to barfing may be intentional.  In his paper about sediment routing systems, he also made the following two statements that cast doubt on the ability of present-day scientists to be sure of anything about the past.

• Ideally, we would know all of the physical and chemical processes governing the sediment-routing system.  This would be enormously gratifying in trying to understand how sediment-routing systems function generically, but we would immediately run into a fundamental problem: the long result of time.  Time transforms sediment-routing systems into geology, and like history, selectively samples from the events that actually happened to create a narrative of what is recorded.  Progress in understanding modern sediment-routing systems now leaves us poised to answer the important question: how do we simultaneously use the modern to generate the time-integrated ancient, and ‘invert’ the ancient to reveal the forcing mechanisms for change in the past?
• Let us take the example of the effects of cyclic glaciation, a mode of response to cyclic climate changes that Earth has experienced in the past few million years (see page 284).  To build a numerical landscape-evolution model for times of glaciation we would need to know the sliding velocity of ice by solution of a chosen ice-dynamics equation, a proportionality constant in the ice-erosion equation that depends on the underlying rock type, a rheological law relating ice deformation to local stress, a model for ice accumulation and ablation, and knowledge of the temperature at the base of the ice.  This might work theoretically by making a large number of assumptions, but the resulting model would be impossible to use in a simulation of Quaternary landscapes.  Why?  Because the necessary parameter values to inform a long-term landscape model are not currently available, and perhaps never will be.  This humbling realization does not denigrate the efforts of modellers working at the human timescale, but instead prompts us to think afresh about what is required for success with upscaled [i.e., extrapolated] models.
      It is not immediately obvious that the factors controlling a long-term response may be different from those controlling local processes.

Allen hoped that interdisciplinary conversations might help shed light on Earth history, but that last sentence sounds like a repudiation of Lyell’s long-esteemed principle of uniformitarianism, “the present is the key to the past.”

1.  Maureen E. Raymo and Peter Huybers, “Unlocking the mysteries of the ice ages,” Nature 451, 284-285 (17 January 2008) | doi:10.1038/nature06589; Published online 16 January 2008.
2.  David J. Stevenson, “A planetary perspective on the deep Earth,” Nature 261-265 (17 January 2008) | doi:10.1038/nature06582.
3.  Phillip A. Allen, “From landscapes into geological history,” Nature 451, 274-276 (17 January 2008) | doi:10.1038/nature06586.

The textbooks don’t tell you how much of natural history is more art than science.  What are the proxy records actually measuring?  How do the orbital explanations translate to actual changes on earth’s surface?  In short, how do they know what they say they know?  It’s nice when the insiders raise the questions.
    Remember, correlation is not the same thing as causation.  Theories are always underdetermined by the data.  There could be very different causes for the observed effects.  These authors maintained a spirit of progressivism in spite of the serious questions they asked.  Earth has no obligation to abide by the current consensus.  For a shocker, read what global warming skeptic Alexander Cockburn thinks of consensus and “scientific” peer review.
    TV documentaries and textbooks often dress up the currently-favored models with glitzy visuals that convey an air of certainty.  Years later, a scientific revolution overturns the paradigm and tosses the old theory out.  So much for the visuals.
    When you watch documentaries, view museum displays or read popular science reports presenting confident-sounding dates of ice ages, etc., remember this little glimpse into the drafting room these insiders gave us.  You’re not watching reality in those programs.  You are watching paradigms.  Like glaciers, paradigms shift, melt, advance, retreat, and sometimes go slip-slidin’ away.