by Judith A. Curry
My invited talk at the American Physical Society Meeting in Denver.
The abstract for my talk:
Causes and implications of the growing divergence between climate model simulations and observations
For the past 15+ years, there has been no increase in global average surface temperature, which has been referred to as a ‘hiatus’ in global warming. By contrast, estimates of expected warming in the first several decades of 21st century made by the IPCC AR4 were 0.2C/decade. This talk summarizes the recent CMIP5 climate model simulation results and comparisons with observational data. The most recent climate model simulations used in the AR5 indicate that the warming stagnation since 1998 is no longer consistent with model projections even at the 2% confidence level. Potential causes for the model-observation discrepancies are discussed. A particular focus of the talk is the role of multi-decadal natural internal variability on the climate variability of the 20th and early 21st centuries. The “stadium wave” climate signal is described, which propagates across the Northern Hemisphere through a network of ocean, ice, and atmospheric circulation regimes that self-organize into a collective tempo. The stadium wave hypothesis provides a plausible explanation for the hiatus in warming and helps explain why climate models did not predict this hiatus. Further, the new hypothesis suggests how long the hiatus might last. Implications of the hiatus are discussed in context of climate model sensitivity to CO2 forcing and attribution of the warming that was observed in the last quarter of the 20th century.
My .ppt presentation is posted here [APS Curry ]. Below is the text from some of the summary slides.
Significance of the pause
Under conditions of anthropogenic greenhouse forcing:
- •Only 2% of climate model simulations produce trends within the observational uncertainty
- Modeled pauses longer than 15 years are rare; the probability of a modeled pause exceeding 20 yrs is vanishing small
Questions raised by the discrepancy
- Are climate models too sensitive to greenhouse forcing?
- Is climate model treatment of natural climate variability inadequate?
- Is the IPCC’s ‘extremely likely’ confidence level regarding anthropogenic attribution since 1950 justified?
- Are climate model projections of 21st century warming too high?
- How confident are we of the observations?
I. Where is the missing heat?
Hypothesis I: It MUST be hiding in the ocean
- Evidence of deep ocean sequestration is indirect; few observations of deep ocean temperature prior to 2005
- Ocean models do not transfer heat in the vertically any where near as efficiently as inferred from the ECMWF reanalyses
- Concerns about the heat returning to the surface seem unrealizable if the heat is well mixed – 2nd law constraints
Hypothesis II: There is NO missing heat; changes in clouds have resulted in more reflection of solar radiation
- Global cloud satellite dataset only goes back to 1983; calibration issues complicate trend analyses
- Global energy balance analyses are associated with significant uncertainties
II. Maybe the models are OK, the problem is the external forcing
There is significant disagreement among different forcing data sets
CMIP5 simulations were forced by single ‘best estimate’ data sets
There has been no systematic effort to assess uncertainty in these data sets or the sensitivity of climate models to forcing uncertainty
These uncertainties have not been factored into the 20th century attribution assessments
III. ENSO (natural internal variability) is masking the greenhouse warming
IV. Multidecadal modes of natural internal variability
- The ‘hiatus’ will continue at least another decade
- Climate models are too sensitive to external forcing
- Hiatus persistence beyond 20 years would support a firm declaration of problems with the climate models
- Incorrect accounting for natural internal variability implies:
- —Biased attribution of 20th century warming
- —Climate models are not useful on decadal time scales
Summary of major uncertainties
- Deep ocean heat content variations and mechanisms of vertical heat transfer between the surface and deep ocean
- Uncertainties associated with external forcing data and implications for attribution analysis and future projections
- Sensitivity of the climate system to external forcing
- Clouds: trends, forcing, feedbacks, and aerosol – cloud interactions
- Nature and mechanisms of multidecadal natural ‘internal’ variability
- Unknowns – solar indirect effects, magnetic and electric field effects, orbital (tidal and other) effects, core-mantle interactions, etc.