Projecting Dr Trenberth and avoiding his question

Which Dr Trenberth?

Anthony Watts, who has written many, many articles about Dr Kevin Trenberth, forgets his name and starts calling him Kenneth (archived - later corrected version).

Source: WUWT

As Frank Kotler says, way way down in the WUWT comments:

August 25, 2013 at 6:13 pm Top post still says “Kenneth” in a couple places. We’re talking about the same guy, right? If we’re going to insist that words have meanings, getting the man’s name right might be a good start…

Predictions and projections

The WUWT-ers are having trouble working out the difference between IPCC model projections and model predictions.  Perhaps they can think of it this way.  IPCC models are used to make projections about climate (surface temperature, precipitation, ice cover etc) for a range of different scenarios.  Scenarios are largely based on emissions trajectories.  That is, if we continue on a very high rate of CO2 emissions, the average global surface temperature will rise much more than if we cut CO2 emissions.  The scenario processes for IPCC AR5 reports are described here.  There are four Representative Concentration Pathways (RCPs) that are described here.  These are:

• RCP8.5 Rising radiative forcing pathway leading to 8.5 W/m2 in 2100.
• RCP6 Stabilization without overshoot pathway to 6 W/m2 at stabilization after 2100
• RCP4.5 Stabilization without overshoot pathway to 4.5 W/m2 at stabilization after 2100
• RCP2.6 Peak in radiative forcing at ~ 3 W/m2 before 2100 and decline

The word "prediction" could be used when referring to a particular scenario as part of an "if" statement.  For example, IF we were to continue to track on or above RCP8.5 I'd predict we'd face a very challenging future!

Jimbo from WUWT posted a comment about an old blog article by Dr Trenberth.  Here is Dr Trenberth's article in full from Nature's climate feedback blog:

Predictions of climate

Posted by Oliver Morton on behalf of Kevin E. Trenberth

I have often seen references to predictions of future climate by the Intergovernmental Panel on Climate Change (IPCC), presumably through the IPCC assessments (the various chapters in the recently completedWorking Group I Fourth Assessment report ican be accessed through this listing). In fact, since the last report it is also often stated that the science is settled or done and now is the time for action.

In fact there are no predictions by IPCC at all. And there never have been. The IPCC instead proffers “what if” projections of future climate that correspond to certain emissions scenarios. There are a number of assumptions that go into these emissions scenarios. They are intended to cover a range of possible self consistent “story lines” that then provide decision makers with information about which paths might be more desirable. But they do not consider many things like the recovery of the ozone layer, for instance, or observed trends in forcing agents. There is no estimate, even probabilistically, as to the likelihood of any emissions scenario and no best guess.

Even if there were, the projections are based on model results that provide differences of the future climate relative to that today. None of the models used by IPCC are initialized to the observed state and none of the climate states in the models correspond even remotely to the current observed climate. In particular, the state of the oceans, sea ice, and soil moisture has no relationship to the observed state at any recent time in any of the IPCC models. There is neither an El Niño sequence nor any Pacific Decadal Oscillation that replicates the recent past; yet these are critical modes of variability that affect Pacific rim countries and beyond. The Atlantic Multidecadal Oscillation, that may depend on the thermohaline circulation and thus ocean currents in the Atlantic, is not set up to match today’s state, but it is a critical component of the Atlantic hurricanes and it undoubtedly affects forecasts for the next decade from Brazil to Europe. Moreover, the starting climate state in several of the models may depart significantly from the real climate owing to model errors. I postulate that regional climate change is impossible to deal with properly unless the models are initialized.

The current projection method works to the extent it does because it utilizes differences from one time to another and the main model bias and systematic errors are thereby subtracted out. This assumes linearity. It works for global forced variations, but it can not work for many aspects of climate, especially those related to the water cycle. For instance, if the current state is one of drought then it is unlikely to get drier, but unrealistic model states and model biases can easily violate such constraints and project drier conditions. Of course one can initialize a climate model, but a biased model will immediately drift back to the model climate and the predicted trends will then be wrong. Therefore the problem of overcoming this shortcoming, and facing up to initializing climate models means not only obtaining sufficient reliable observations of all aspects of the climate system, but also overcoming model biases. So this is a major challenge.

The IPCC report makes it clear that there is a substantial future commitment to further climate change even if we could stabilize atmospheric concentrations of greenhouse gases. And the commitment is even greater given that the best we can realistically hope for in the near term is to perhaps stabilize emissions, which means increases in concentrations of long-lived greenhouse gases indefinitely into the future. Thus future climate change is guaranteed.

So if the science is settled, then what are we planning for and adapting to? A consensus has emerged that “warming of the climate system is unequivocal” to quote the 2007 IPCC Fourth Assessment Working Group I Summary for Policy Makers (pdf) and the science is convincing that humans are the cause. Hence mitigation of the problem: stopping or slowing greenhouse gas emissions into the atmosphere is essential. The science is clear in this respect.

However, the science is not done because we do not have reliable or regional predictions of climate. But we need them. Indeed it is an imperative! So the science is just beginning. Beginning, that is, to face up to the challenge of building a climate information system that tracks the current climate and the agents of change, that initializes models and makes predictions, and that provides useful climate information on many time scales regionally and tailored to many sectoral needs.

We will adapt to climate change. The question is whether it will be planned or not? How disruptive and how much loss of life will there be because we did not adequately plan for the climate changes that are already occurring?

Kevin Trenberth
Climate Analysis Section, NCAR

The Question

The question is not about predictions vs projections. As Dr Trenberth writes in his closing paragraph: We will adapt to climate change. The question is whether it will be planned or not?

This is the global CO2 emissions from fossil fuel and cement production (not including land use change or deforestation) up to and including 2011 from a 2012 report by the PBL Netherlands Environmental Assessment Agency:

Source: Trends in global co2 emissions 2012 Report, PBL Netherlands Environmental Assessment Agency

Click here for an interactive chart in the Guardian, which you can play with.  It shows CO2 emissions from around the world by country, per GDP, per capita etc.


So while WUWT-ers are arguing over whether or not the IPCC makes predictions or is careful with terminology, the world continues to pour carbon dioxide into the air.  We need to drop emissions, by a lot, if we are to have any chance of limiting global warming to just two degrees.