by Judith Curry
Time of emergence of a warming signal is a topic that is receiving increasing discussion.
The IPCC AR5 discusses this in section 18.104.22.168.2. I don’t particularly suggest reading the AR5 text, since it is typically painful to read.
Ed Hawkins has blogged on this Time of emergence of a warming signal, with a nice summary. Some excerpts:
The ‘signal’ of a warming climate is emerging against a background ‘noise’ of natural internal variability. Both the magnitude of the signal and the noise vary spatially and seasonally. As society and ecosystems tend to be somewhat adapted to natural variability, some of the impacts of any change will be felt when the signal becomes large relative to the noise.
The concept of a ‘time of emergence’ (ToE) has been discussed by several authors and the IPCC AR5 includes a section (22.214.171.124.2) on when and where the temperature signal will emerge. Each model’s ToE is defined using the standard deviation from a control simulation as the noise (N), and the temperature change since the mean of 1986-2005 as the signal (S).
[Fig. 11.11 from the IPCC report; reproduced in Ed Hawkins' post] shows maps of the median year in which S/N becomes larger than 1. The darker red colours, indicating earlier ToE, are found in the tropics. Even though the signal of change is not largest in the tropics, the variability tends to be smaller than at higher latitudes, thus giving earlier emergence times. The histograms illustrate the uncertainty in ToE for particular area averages, which is due to uncertainty in both the signal and the noise in different models.
A new paper is published today in Nature that provides a different perspective on this issue.
The projected timing of climate departure from recent variability
Camilo Mora, Abby G. Frazier, Ryan J. Longman, Rachel S. Dacks, Maya M.Walton, Eric J. Tong, Joseph J. Sanchez, Lauren R. Kaiser, Yuko O. Stender, James M. Anderson, Christine M. Ambrosino, Iria Fernandez-Silva, Louise M. Giuseffi & ThomasW. Giambelluca
Ecological and societal disruptions by modern climate change are critically determined by the time frame over which climates shift beyond historical analogues. Here we present a new index of the year when the projected mean climate of a given locationmoves to a state continuously outside the bounds of historical variability under alternative greenhouse gas emissions scenarios. Using 1860 to 2005 as the historical period, this index has a global mean of 2069 (618 years s.d.) for near-surface air temperature under an emissions stabilization scenario and 2047 (614 years s.d.) under a ‘business-as-usual’ scenario. Unprecedented climates will occur earliest in the tropics and among low-income countries, highlighting the vulnerability of global biodiversity and the limited governmental capacity to respond to the impacts of climate change. Our findings shed light on the urgency of mitigating greenhouse gas emissions if climates potentially harmful to biodiversity and society are to be prevented.
Phys.org has an article on this new paper titled: Study reveals urgent new time frame for climate change. Excerpts:
The new index shows a surprising result. Areas in the tropics are projected to experience unprecedented climates first – within the next decade. Under a business-as-usual scenario, the index shows the average location on Earth will experience a radically different climate by 2047. Under an alternate scenario with greenhouse gas emissions stabilization, the global mean climate departure will be 2069.
“The results shocked us. Regardless of the scenario, changes will be coming soon,” said lead author Camilo Mora. “Within my generation, whatever climate we were used to will be a thing of the past.”
Tropical species are unaccustomed to climate variability and are therefore more vulnerable to relatively small changes. The tropics hold the world’s greatest diversity of marine and terrestrial species and will experience unprecedented climates some 10 years earlier than anywhere else on Earth. Previous studies have already shown that corals and other tropical species are currently living in areas near their physiological limits. The study suggests that conservation planning could be undermined as protected areas will face unprecedented climates just as early and because most centers of high species diversity are located in developing countries.
“This paper is unusually important. It builds on earlier work but brings the biological and human consequences into sharper focus,” said Jane Lubchenco, former Administrator of the National Oceanic and Atmospheric Administration and now of Oregon State University, who was not involved in this study. “It connects the dots between climate models and impacts to biodiversity in a stunningly fresh way, and it has sobering ramifications for species and people.”
JC comments: I like the methodology used in the new study (well, apart from the issue of the climate models running too hot and not simulating natural internal variability very well). To me, it makes much more sense to look at the historical variability since 1860, than merely to look at the period 1986-2005. Further, I like this approach better than the detection and attribution approaches used by the IPCC, whereby simulations forced with only natural variability are compared with simulations with natural plus anthropogenic variability. Since the models do a poor job simulating natural internal variability, it seems a much better strategy to look at the regional natural variability from the historical data record.
With regards to Mora et al. being “shocked” by their results, well their results seem to me to be far less alarming than the results reported by the AR5, whereby Mora et al. find emergence of the signal several decades later than reported by the AR5.
The issue of the tropics emerging as the key ‘hot spot’ for emergence is interesting, since the amount of warming expected for the tropics is substantially less than for the high latitudes. I can’t speak with any great understanding regarding the adaptability of tropical ecosystems to this relatively small amount of warming. But in terms of adaptation of human societies, the main issue is fresh water availability – an issue for which the climate models show little skill, and for which population increase is the main driver.
So, sounding the ‘alarm’ based on this paper seems misguided, since this paper seems markedly less alarming than what was reported in the AR5.
And finally, how do we square this concept of ‘time of emergence’ with the statement from the IPCC AR5 Report, chapter 10: “On every continental region except Antarctica, it is likely that anthropogenic influence has made a substantial contribution to surface temperature increases since the mid 20th century.”