“Projection

The term “projection” is used in two senses in the climate change literature. In general usage, a projection can be regarded as any description of the future and the pathway leading to it. However, a more specific interpretation has been attached to the term “climate projection” by the IPCC when referring to model-derived estimates of future climate.

Forecast/Prediction

When a projection is branded “most likely” it becomes a forecast or prediction. A forecast is often obtained using deterministic models, possibly a set of these, outputs of which can enable some level of confidence to be attached to projections.

Scenario

A scenario is a coherent, internally consistent and plausible description of a possible future state of the world. It is not a forecast; rather, each scenario is one alternative image of how the future can unfold. A projection may serve as the raw material for a scenario, but scenarios often require additional information (e.g., about baseline conditions). A set of scenarios is often adopted to reflect, as well as possible, the range of uncertainty in projections. Other terms that have been used as synonyms for scenario are “characterisation”, “storyline” and “construction”.”

True that scenarios don’t, but projections are predictions under certain conditions(e.g when the words “most likely” are attached). See definitions pages at IPCC and WMO.

He made that very clear in the introduction:
“In this paper we study the response of a 3D global climate model to realistic rates of change of radiative forcing mechanisms.”

Response to forcing. That’s what his analysis calculates. Not airborne fraction etc. And he’s entitled to assume that readers of that journal would understand that.

Read my posting above. BAU [wrt CO2] (Scenario A) called for an annual increase in anthro emissions of CO2 of 1.5%.
Source: Global Climate Changes… Hansen et al 1988

“Ch. 4.1 Trace Gases
Scenario A assumes that growth rates of trace gas emissions typical of the 1970s and 1980s will continue indefinitely; the assumed annual growth averages about 1.5% of current emissions, so the net greenhouse forcing increasese xponentially.”

Instead, from 1990-2010, human emissions went up 45%, or 1.9% p.a.
Source: http://edgar.jrc.ec.europa.eu/CO2REPORT2012.pdf
Table A.1.2 P28

Kurt in Switzerland

What is it about the term “emissions” which you don’t comprehend?

There are two poorly-understood relationships regarding Carbon Dioxide:

1) how emissions affect the atmospheric concentration (short and long term)
2) how atmospheric concentration affects “global climate”

Granted, #2 is the sexier of the two (and gets most of the attention).

But the public policy discussion is all about emissions (and not about concentration). And both are important to complete the picture.

My point (summarized well by Steve) is that Hansen got it very wrong on the relationship between the human emissions and the climate; this involves steps 1 and 2 above.

Clear now?

Kurt in Switzerland

But in terms of the functionality of Hansen’s model, this is not relevant. That model works out the temperature response to a forcing. The time history of the forcing is the scenario input. The model was not expected to be able to determine airborne fraction. It doesn’t even add knowledge of the relation between gas ppm and forcing in watts/m2; for that it uses published empirical relations.

Reasonable people can disagree on the meaning of Scenario B in terms of CO2 emissions since it is expressed in ppm. My guess is that CO2 emissions in tonnes would rise more steeply than Scenario B if plotted against the Hansen graphic. Collating CO2 emission numbers on a consistent basis requires some care in data handling. I’ve looked at this a little, but not posted on it. I’ll do this exercise some time.