Geoscience Reference
In-Depth Information
The Sun may vary by as much as 0.1 percent over the 11-year solar cycle and perhaps by 0.05 to
0.3 percent over centennial timescales. Uncertainty in the long-term variability is limited by the proxy
models used to derive irradiance backward in time, before the measurement record began. Kopp used an
estimate of how the Sun might have varied coming out of the Maunder Minimum, by approximately 0.1
percent over 80 years, to derive accuracy and stability requirements for measuring TSI. Although change
of that magnitude may be easy to measure over a solar rotation or over a maximum-to-minimum half
solar cycle, it is a considerable challenge to derive a change that small over a century. This derivation
requires a stability of 10 parts per million (which is at the frontier of the possible with the best
instruments today), and it requires substantial periods of overlap between those measurements with
different instruments and a long-term accuracy of 100 parts per million. These tight demands make it
difficult to obtain such measurements needed for climate science. To resolve the offsets among the
various measurements of TSI that make up the 33-year record, Kopp and his colleagues established a test
facility using a calibrated cryogenic radiometer and a laser source at solar power levels to provide the
first-ever end-to-end validation of active cavity solar irradiance instruments. This facility also helped
resolve the source of the offsets among the various TSI instruments. For the SORCE Total Irradiance
Monitor (TIM) the precision aperture is at the front of the instrument. All of the other sensors use view-
limiting apertures in front of the precision aperture. By overfilling and underfilling that aperture, Kopp
determined that scattered light was the source of a large fraction of the offsets between those instruments
and SORCE TIM, which measures a TSI of approximately 1361 W m -2 , the lowest among the group (see
Figure 1.1 in Chapter 1). The scattering-corrected ACRIM measurements are now very close to this
value, as are those from PREMOS. A large scattering correction was derived for the VIRGO instrument,
but it has yet to be applied.
Although the general agreement among the ACRIM, PREMOS, and TIM instruments has
improved since the discovery of scattered light in the ACRIM and PREMOS sensors, Kopp showed that
only the TIM is accurate (100 ppm) and stable (10 ppm per year) enough to monitor the long-term
changes of the Sun at climate-quality levels on timescales of years to decades. Without this level of
stability it is difficult to distinguish real solar change (for example, between successive solar minima)
over instrument drift. Thus, Kopp concluded, it is crucial that the TSI record from TIM remains
unbroken, a proposition growing riskier with time since the failure of the Glory launch in 2011, and one
that now must rely on overlap between SORCE and the Total Solar Irradiance Sensor.
Assessing Solar and Solar-Terrestrial Influences as a Component of Earth's
Climate Change Picture
Daniel N. Baker, University of Colorado, Boulder
Given the previous discussion there are then two challenges: understanding the variation in TSI in
the modern era and extrapolating back in time on the order of tens of millennia to understand the variation
in TSI through the use of proxies. Baker pointed out that measurements of total solar irradiance vary
widely, and the normalization of the values could possibly obscure small trends—a problem he feels
should be addressed. Historical TSI reconstruction connects these contemporary TSI measurements via
an index that requires extrapolating the TSI back in time—with the attendant uncertainties. As Baker
summarized, the connection between TSI and various proxies is that the size of the heliosphere controls
the number of galactic cosmic rays (GCRs) that reach Earth: the GCR flux is higher at solar minimum.
Isotopic abundances in the atmosphere are altered by GCR flux, generating increased 14 C and 10 Be, and
this isotopic evidence is found in tree rings and ice cores.
Baker also noted that GCRs reach into the stratosphere and troposphere, and are an important
“top-down” mechanism for coupling the Sun to climate. For example, Lockwood et al. (2006) found a
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