Paleoclimate: The state of our knowledge

Assessment

 

As the IPCC is likely to begin work on another assessment report (AR6), it is important to look critically at how well AR5 achieved its goals of summarizing and evaluating the state-of-the-art knowledge surrounding climate change. The following is my personal opinion of the strengths and weaknesses of chapter 5. 

 

How Chapter 5 fits in with the full AR5:

 

A separate paleoclimate chapter was not included in the first 3 IPCC Assessment Reports. This does not mean that no paleoclimate information was included in those reports--instead, pertinent paleoclimate data were scattered throughout the report where deemed relevant. By creating a full, separate chapter for paleoclimate, the IPCC ensures that a more full treatment of the field is included. However, much of the information in chapter 5 would be useful in other chapters. For example, a more complete treatment of past sea level rise would be helpful in providing context in chapter 13 on sea level rise (as this chapter currently focuses on industrial-era and future projections of sea level rise). Some debate still exists over whether the paleoclimate chapter should exist as a stand-alone chapter or if it should be spread throughout the report; of course, this debate is only relevant if an AR6 happens, which has not yet been confirmed. My opinion is that there should be a stand-alone chapter AND a better treatment of paleo-data in each of the other chapters. 

 

What is done well:

 

• The IPCC has focused on appropriate periods in Earth's history to provide a sense for (1) some of the extremes in Earth’s history, particularly very warm times that may serve as analogues for the future, and (2) more recent paleoclimate (i.e. the Holocene), which is represented by a richer set of climate archives and is more directly comparable to the present  because of orbital parameters, tectonic configuration, vegetation cover, and more.

• This assessment report (AR5) did a good job of focusing on improvements over the last report (AR4), rather than rehashing all of the details of what we already knew a few years ago.

• The authors appropriately focused on the most pertinent climate metrics and processes to modern climate change—like temperature change, sea level rise, climate sensitivity, and abrupt climate change.

• The IPCC assessment reports have been described as encyclopedic, and if treated that way, this chapter is organized in a way that it is relatively easy to look up information. See below for more comments on organization. 

• The authors did well paring down a massive pool of information and highlighting the important research in this chapter. There is always more that could be covered, but interested readers can seek that information in the cited literature. 

• In general (but not always), the authors did a good job of defining specific paleoclimate events and explaining their significance. Paleoclimate is a jargon-heavy discipline, and the names of geologic periods and events can be daunting to keep straight; the summary table and in-text explanations are helpful in this regard. 

 

What needs improvement:

 

• The organization of the chapter is not chronological and is thus confusing for readers who are not well-versed in the names/characteristics of paleoclimate events. Perhaps a better organizational system would be to highlight characteristics of each time period starting with the oldest and moving forward in time.

• Many of the figures are trying to do too much and require too much expertise in order to glean any information. Instead of cramming a large amount of information (from various regions, proxies, metrics, etc.) into a small number of figures, it would be more beneficial to the reader to have fewer figures that are each more straightforward to interpret.