Overview of the various models for ice ages in the recent past (3 MYBP to present) - Ice Ages and Interglacials: Measurements, Interpretation, and Models - page 234

however, is that the short period involved in the study is statistically inadequate to
draw firm conclusions.
Svensmark (2000) extended previous work. He showed that the production of
radiocarbon-14 in the Earth's atmosphere was inversely related to the pattern of
Earth temperature over the past 1,000 years, with low production of 14 C during
the MWP and high production during the LIA. The production of 14 C decreased
sharply in the 20th century along with global warming. All of this is compatible
with Figures 11.10 and 11.11 . Svensmark said:
''In 1900 the cosmic rays were generally more intense than now and most of
the warming during the 20th Century can be explained by a reduction in low
cloud cover. Going back to 1700 and the even higher intensities of cosmic rays,
the world must have seemed quite gloomy as well as chilly, with all the extra
low-level clouds.''
Lockwood and Fro¨ hlich (2007) published a rebuttal to Svensmark's theory.
They admitted that over the 20th century the trend in 10 Be has been downward
and the temperature trend upward, which supports Svensmark's theory. However,
they claimed:
''Over the past 20 years, all the trends in the Sun that could have had an
influence on the Earth's climate have been in the opposite direction to that
required to explain the observed rise in global mean temperatures.''
Svensmark and Friis-Christensen (2007) responded to Lockwood and Fro¨ hlich
(2007) by pointing out that the use of running means of global temperature data
over about 10 years obfuscated the fact that temperatures stopped rising after
1998. In addition, discrepancies between tropospheric temperature trends and
surface temperature trends led to different conclusions on temperature variations
over the past few decades. Using tropospheric temperatures without averaging and
allowing for the effects of El Nin ˜ os and volcanic eruptions, Svensmark and Friis-
Christensen (2007) found a good anti-correlation between cosmic ray levels and
global temperatures over the past few decades. It is also noteworthy that the bias
of observers toward (or against) the alarmist position on global warming produced
by CO 2 may have crept into the arguments. Lockwood and Fro¨ hlich (2007)
proclaimed the alarmist position with apparent satisfaction:
''Our results show that the observed rapid rise in global mean temperatures
seen after 1985 cannot be ascribed to solar variability, whichever of the
mechanisms is invoked and no matter how much the solar variation is
amplified.''
Svensmark and Friis-Christensen (2007) took the opposite position:
''The continuing rapid increase in carbon dioxide concentrations during the