If you are a follower of TV crime shows, it is likely that you’ve come across one of the CSI offshoots (CSI stands for Crime Scene Investigation) and a slightly less well known show called ‘Cold Case‘. In both these shows, difficult crimes (usually murders) are solved using the most up-to-date forensic methods and incredible detective work. However, it will be obvious to even the most jaded TV watcher that the CSI crew get to have a lot more fun with the latest gadgets and methodologies. The reason for that is clear: with a fresh crime scene there is a lot more evidence around and a lot more techniques that can be brought to bear on the problem. In a ‘Cold Case’ (where the incident happened years before), options are much more limited.
Why bring this up here? Well it illustrates nicely how paleo-climate research fits in to our understanding of current changes. Let me explain….
For the last 30 years or so, the amount of information we have about the planet has gone up by a couple of orders of magnitude – mainly due to satellite information on atmospheric (radiation, temperature, humidty, rainfall, cloudiness, composition etc.), ocean surface (temperature, ice cover, windiness) and land properties (land cover, albedo) etc. Below the surface, we are now measuring much more of the ocean changes in heat content and carbon. This data, while still imperfect, has transformed our view of the climate such that the scientists studying it can seriously discuss details of problems that twenty years ago were not even thought of as issues. “CSI – Planet Earth” if you like.
Comparatively, the amount of information we have for any period in the past is less – hundreds (in some cases a few thousand) of records of climate ‘proxy’ data (i.e. records that are related to climate, such as tree rings ot isotope ratios, but that aren’t direct thermometers or rain gauges) that are not necessarily optimally spaced, nor necessarily well-dated, nor uncontaminated by non-climate influences. However, there is the great advantage of a much longer time period to work with, as well as a greater variety of changes to investigate. Think of the people that work on that as the ‘Cold Case’ crew.
The most prevalent reasonably scientific question about current climate changes is ‘how do we know that this isn’t natural variability?’. A number of versions of that question came up in the House hearing last week (a nice report from the proceedings can be found here). Some of those comments were serious, some were ridiculous, but all essentially pointed to the same issue. Kevin Trenberth and Richard Alley answered it best when they pointed out that the causes of ‘natural variability’ – whether the sun, volcanoes or ocean changes – should be detectable (but haven’t been), and that the anthropogenic ‘hypothesis’ should have consequences that are also detectable (which have). Add in the modelling studies which indicate that current conditions can’t be explained without including greenhouse gases and you have a pretty solid case that what is happening is in large part anthropogenic.
A rather more specious comment heard often (including at this hearing) is that ‘if it was warmer before, then the current warming must be natural’ or alternatively ‘if you can’t explain all of the past changes, how can you explain anything now?’. First of all, there are many periods in Earth history that are unequivocally accepted to be warmer than the present – the Pliocene (3 million years ago), the Eocene (50 million years ago) and the mid-Cretaceous (100 million years ago) for instance. Less clearly, the Eemian interglacial period or the Early Holocene may have been slightly warmer than today. Thus, if that logic were appropriate, no-one should bother worrying about climate change until sea levels start to approach mid-Cretaceous levels (about 100m above today’s level!).
However, the logic is fatally flawed. It is akin to a defense lawyer arguing that their client can’t possibly have committed a particular murder because other murders have happened in the past that were nothing to do with them. That would get short shrift in a courtroom, and the analgous point gets short shrift in the scientific community too. Of course, it is possible that our suspect was involved in previous murders too – but obviously the further back you go, the harder it is to pin it on them. And clearly, there will be past murders where they have a clear alibi.
A better tactic for the defense is obviously to try and pin it on someone else – and if that someone else has a record – then all the better. Therefore, ‘the sun did it’ is a frequent accusation, but as we have discussed here quite often, this time around the sun has an alibi and there are reliable witnesses to back him up.
Given the better information and resources available for the CSI crew, it is natural that their assessment of the current case will generally hold sway. Cold Cases (or paleo-climate) are of course of paramount interest: they provide a much wider set of conditions that set the stage for the modern analyses and provide plenty of test cases for us to hone our techniques (such as climate modelling). However arguments from paleo are extremely unlikely to trump the modern analyses – whether they refer to the medieval warm period or the Phanerozoic.
So to summarise, CSI-Planet Earth have a good case for pinning the latest warming on greenhouse gases. Cold Case has evidence that they were involved in some previous cases (the last glacial period for instance), though they’ve definitely ruled our suspect out for a few others (e.g. the 8.2kyr event). It would be hard to argue that our suspect should be acquitted because there have been some crimes they didn’t commit!
Update: I should have linked to this Newsday piece: Hot on their global trail by Bryn Nelson where I first tried out this analogy.