Climate projections don’t include volcanic eruptions.

via Tallbloke’s Talkshop
http://ift.tt/1WIzElD

chile-volcano In today’s climate models, volcanic eruptions are thought to be a major negative forcing (they have to be, to counteract the over-egged positive forcing attributed to CO2), but the ‘projections’, ‘forecasts’, ‘scenarios’ (anything but predictions!) don’t include any volcanic eruptions.

That being the case, why would we take any notice of model output which ‘projects’ large warming in the future?

Judith Curry:
Under the RCP8.5 scenario, the CMIP5 climate models project continued warming through the 21^st century that is expected to surpass the ‘dangerous’ threshold of 2°C warming as early as 2040. It is important to note that the CMIP5 simulations only consider scenarios of future greenhouse gas emissions – they do not include consideration of scenarios of future volcanic eruptions…

Thayer Watkins
There is a distinction between forecasts and projections. A forecasting model or method attempts to predict what actually happens on the basis of information known before it happens. A projection, on the other hand, says what will happen under a set of assumptions. Some of the assumptions are reasonable and necessary such as that there will be no major volcanic eruptions during the period over which the projections are made. Such assumptions have to be made for forecasts as well.

Wikipedia
Future scenarios do not include unknown events – for example, volcanic eruptions or changes in solar forcing. These effects are believed to be small in comparison to greenhouse gas (GHG) forcing in the long term, but large volcanic eruptions, for example, can exert a substantial temporary cooling effect.

The Guardian
The measured surface warming has been about 0.13°C less than the average of model simulations since 2000. The estimated volcanic cooling from this new paper (0.05–0.12°C), not included in those climate models, could account for most of that discrepancy.