Fingerprints of Solar Irradiance Changes
during the Last Millennium
Impact of Different Background Trends on the Detection in Transient
Climate Simulations
Caspar Ammann, Hee-Seok Oh, Philippe Naveau
Also: Fortunat Joos (University of Bern, Switzerland),
Dave Schimel, Bette OttoBliesner (NCAR)
National Center for Atmospheric Research
Climate and Global Dynamics Division
Paleoclimatology
Boulder, Colorado (USA)
20th Century Experiments
Anthropogenic forcing
CSM
Natural External Forcings
Volcanic Aerosol
Solar Irradiance Changes
20th Century Experiments
Natural only and combined natural-anthropogenic
IPCC 2001
“In the light of new evidence and
taking into account the remaining
uncertainties, most of the observed
warming over the last 50 years is
likely to have been due to the
increase in greenhouse gas
concentrations.”
Humans, Nature and Climate
“Of the rise in temperature during the 20th century, the bulk occurred from
1900 to 1940. It was followed by the … cooling trend from 1940 to around 1975.
Yet the concentration of greenhouse gases was measurably higher in that later
period than in the former. ”
“When we look back over the past millennium, the questions that arise are even
more perplexing. The so-called Climatic Optimum of the early Middle Ages,
when the earth temperatures were 1 to 2 degrees warmer than today and the
Vikings established their flourishing colonies in Greenland, was succeeded by
the Little Ice Age, lasting down to the early 19th century. Neither can be
explained by concentrations of greenhouse gases.”
“Most significant: The possibility of long-term cycles in solar activity is
neglected because there is a scarcity of direct measurement. Nonetheless,
solar irradiance and its variation seem highly likely to be a principal cause of
long-term climatic change.”
James Schlesinger, Former Secretary of Energy
Monday, July 7, 2003
Solar Irradiance Estimates:
Beryllium Record (Bard et al. 2000)
Wolf
Oort
Maunder
Spörer
“1900”
Dalton
Forcings for Millennium Simulations
Model / Data Intercomparison
Modeling
Uncertainties:
- Real Forcing
- Climate Sensitivity
PaleoClimate
Reconstruction
Uncertainties:
- Forced vs Internal
- Spatial Extent
CSM 1.4 Millennium Simulations
Climate of the last 1800 years
Northern Hemisphere Annual Temperature
Mann et al. 2003
Comparison “Lean forced” CSM
with Paleoclimate Reconstructions:
Only clear difference is to Mann et
al. (1999), which is warmer ~1450-1720
Jones et al. (1998): is mostly summer season.
Difference: < 0.1 C
Crowley & Lowery (1999): Only 11-year
average. Difference: very small
Briffa et al. (2001): exclusively tree-rings, but
with low-freq. retention:
somewhat warmer 1400-1500.
Huang et al. 2000: Subsurface (borehole) temperatures: ~1 C:
Multi-Resolution Method:
See Oh et al. 2003
Solar Low-Frequency Properties:
Proxy Data
>> There seems to be a low-frequency variation in climate
that follows the prescribed solar irradiance estimate
despite missing understanding of exact mechanism
<<
Last Millennim Runs:
Correlations Runs and Reconstructions
Correlations yrs 1400-1849
CSM Low Sol
CSM Low Sol
CSM High Sol
Mann
Jones
Briffa
Crowley
CSM High Sol
0.804
Mann
0.814
0.637
Jones
0.543
0.291
0.681
Briffa
0.286
0.032
0.440
0.738
0.804
0.814
0.543
0.286
0.453
0.637
0.291
0.032
0.151
0.681
0.440
0.672
0.738
0.751
0.760
0.580
0.383
0.649
0.601
0.451
Crowley
0.453
0.151
0.672
0.751
0.760
0.558
Contributions from External Forcing
over the Millennium (850-1999)
(explained variance “Lean-scaled run”)
Solar Forcing
Volcanic Forcing
Raw Data
Low Frequency
(> 200 yrs)
Raw Data
Raw Data (14001850)
Low Frequency
(> 200yrs)
31.9%
43.5%
20.1%
27.2%
4%
Last Millennim Runs:
Solar Signal : Pre-Industrial and 20th Century
No background trend in solar irradiance?
Implications on Climate Sensitivity?
IPCC 2001
“…natural forcings may have contributed
to the observed warming in the first
half of the 20th century. … (But) Changes
in natural forcing during most of (the
last 50 years) are also estimated to be
negative and are unlikely to explain the
warming.”
“In the light of new evidence and taking
into account the remaining uncertainties,
most of the observed warming over the
last 50 years is likely to have been due
to the increase in greenhouse gas
concentrations.”
Conclusions I
•
We can find a low-frequency fingerprint consistent with Beryllium
based solar activity reconstructions in paleo climate records over
the past Millennium.
•
The coupled NCAR-CSM can reproduce these low-frequency
variations. Lack of background trends probably still produce a small
signal, but the relative role of volcanoes would be increasing in the
low-frequency part.
•
High-, medium or low-background trends don’t cause significant
differences in the 20th century where after 1970 GHG changes
dominate the climate.
•
High-frequency signals are very ambiguous, independent of
instrumental or proxy based, because of internal modes of climate
with similar time scales. This does not exclude some areas possibly
showing a correlation, but it is very weak at best.
Conclusions II
Given a model climate sensitivity of 2°C (2xCO2):
•
smaller rather than larger solar irradiance changes are most
consistent with the paleo record.
•
Some background trend seams possible if ozone feedbacks don’t
significantly enhance the response.
•
If the real world climate sensitivity is larger, then a only a very
small or even no background trend would produce the best fit to
the paleo data.
Since we don’t neglect long-term cycles in solar activity and given
the reasonable reproduction of the last Millennium, what can we
expect in the near future?
Possible Outlook …
(IPCC A2 scenario)