The impact of solar irradiance variations on Earth’s surface climate has been debated by many in the past. Based on correlations between solar variability and meteorological changes, the Sun-climate link seems obvious but, as is often stated, correlation does not prove causation. Previously, any link was disputed because the amount of energy delivered by the Sun was deemed too small to have a significant impact. New satellite measurements indicate that variations in solar ultraviolet irradiance may be larger than previously thought, forcing a reevaluation of the impact of solar variation. A recent report in the journal Nature Geoscience claims to show just that—a link between the 11 year solar cycle and Northern Hemisphere winters.
Using older measurements of solar variability over the 11 year solar cycle as input, climate models have proven incapable of establishing linkage between insolation and climate. Still, there have been tantalizing reports of such linkage in the past (see “The Sun's Hidden Power”). In a report in the August 28, 2009, issue of the journal Science entitled “Amplifying the Pacific Climate System Response to a Small 11-Year Solar Cycle Forcing,” Gerald A. Meehl et al. described a possible mechanism that could explain how seemingly small changes in solar output can have a big impact on Earth's climate. Their work explained how the upper atmosphere can act as a solar heat amplifier when UV radiation from the Sun increases.
Now, new, more accurate measurements taken by satellites have revised the amount of variability in insolation, particularly in the ultraviolet frequencies. In “Solar forcing of winter climate variability in the Northern Hemisphere,” Sarah Ineson et al. have applied these new data to a revised climate model and report positive linkage between insolation variability and climate. The researchers explain the importance of the new satellite data.
Satellite observations of solar spectral irradiance in the ultraviolet region have been subject to uncertainty; the Solar Stellar Irradiance Comparison Experiment and Spectral Irradiance Monitor (SIM) instruments aboard the Solar Radiation and Climate Experiment (SORCE) satellite mission (2004–present) are the first designed to achieve accurate long-term measurements of the solar irradiance variations over the entire ultraviolet range. The 200–320 nm part of the ultraviolet band contributes strongly to solar heating in the middle atmosphere, largely through ozone absorption. Ozone is itself produced through the interaction between ultraviolet radiation and oxygen, giving rise to potential positive feedback. SORCE observations made during the decline of solar cycle 23 reveal a remarkably strong decrease in mid-ultraviolet flux, some four to six times greater than previous spectral irradiance reconstructions. However, before the SORCE mission, variations at these wavelengths were poorly constrained, with measurement uncertainty exceeding the potential solar-cycle variation.
The researchers found that their updated model responded to the solar minimum with patterns in surface pressure and temperature that resemble changes in a periodic climate cycle called the North Atlantic or Arctic Oscillation. When the Sun's output dipped, the NAO displayed corresponding winter variation, entering its negative phase. Further more, the variations were of similar magnitude to observations. “In our model, the anomalies descend through the depth of the extratropical winter atmosphere,” they state. “ If the updated measurements of solar ultraviolet irradiance are correct, low solar activity, as observed during recent years, drives cold winters in northern Europe and the United States, and mild winters over southern Europe and Canada.”
During the winter months—December to February—the simulated and observed response at the solar minimum is reflected in substantial changes over the whole northern hemisphere. The model output corresponds to a negative Arctic Oscillation or North Atlantic Oscillation-like pattern (AO/NAO), with sea-level pressure increases at high northern latitudes and decreases at mid-latitudes in both the Pacific and Atlantic basins. This model response is in good agreement with the observed response, although some observational uncertainty in the Atlantic basin remains.
Difference in winter surface climate for solar minimum minus solar maximum.
Both measured and predicted sea-level pressure and near-surface temperatures are shown in the figure above. Quantifying the change in the AO sea-level pressure difference between mid-latitudes and the Arctic gives a shift of −1.2 hPa for the model, which is in good agreement with −1.1 hPa for the reanalysis. As would be expected from the model surface pressure pattern, decreased westerly flow in the Atlantic sector leads to cold near-surface temperatures (figure c) over northeastern Europe and northern Asia and mild conditions further south. The authors claim that this is in reasonable agreement with observations (figure d), which also show negative anomalies extending over much of northern Eurasia.
The researchers concluded that the surface response to the stronger UV forcing in the middle atmosphere is substantially greater than previously calculated. “The solar effect presented here contributes a substantial fraction of typical year-to-year variations in near-surface circulation, with shifts of up to 50% of the interannual variability,” the author's state. The simulated response at Earth's surface resembles the negative phase of the NAO during solar minimum and is of similar magnitude to observed differences in surface climate between solar minimum and solar maximum conditions. These results are consistent with the observed cold winters in northern Europe and the US, and mild winters over southern Europe and Canada in 2008/9, 2009/10 and 2010/11. In other words, driven by the new insolation data the model is in agreement with observed reality.
Even so, the paper's results are not without their critics. Writing in a news & views article in the same issue of Nature Geoscience, Katja Matthes from the GFZ German Research Centre for Geosciences, offers the following critique:
These results are intriguing, but as always there are a number of caveats. First, the study neglected any effects of solar variability on ozone chemistry, which would probably amplify the signal. Second, the simulations consider only the limited UV wavelength range, which is in phase with the solar cycle, but not the visible or the near-infrared wavelengths. In the visible and near-infrared range, SIM measurements are out of phase with the solar cycle, contrary to previous understanding of 11-year solar variability. The SIM data have been used in a radiative-photochemical model and in climate models to assess the atmospheric implications of the radiation changes across the entire spectrum of wavelengths, but without explicit simulation of full ocean dynamics. However, these simulations did not attempt to quantify the impact of changes in solar activity on surface climate in the region under the influence of the NAO. Finally, the trends seen in the SIM observations are still under discussion and remain to be confirmed.
The final chapter has not been written regarding the Sun-Climate link. Currently limited data prevents further conclusive calculations—accurate satellite data covers less than one solar cycle. Other researchers have expressed reservations concerning accuracy and applicability of the SIM data to other solar cycles. But the message seems clear, the Sun's output varies more than scientists previously thought and that variation can have direct impact on Earth's climate. What's more, there are longer term cycles in solar output which could be a major driver of climate change over time scales much longer than decades.
There are a number of lessons to be learned from the Sun-Climate saga. First, claims made by scientists must always be taken in the context of “based on the best data available at the time.” When the IPCC reports discounted insolation variability as a major driver of climate change the real nature of solar variability was not known. Second, better data can significantly change science's accepted view of nature. The new satellite data has forced a reevaluation of the 11 year solar cycle and how insolation at different frequencies change. Third, when models are fed better data, they give better answers. Which raises the question of how accurate the older model predictions are, since they did not incorporate these new data.
Finally, it is hard for scientists to let go of an old theory, even when it is shown to be incorrect. This work and others have shown that the Earth climate system is much more sensitive to changes in our local star. Even so, it will take more satellite data, collected over a number of solar cycles, to conclusively prove just how important solar variability is. Until that comes to pass, climate alarmists will continue to blame everything under the Sun on CO2 emissions.
Be safe, enjoy the interglacial and stay skeptical.