Several times in recent weeks people have commented that the sun is responsible for the current climate change rather than CO2. Ironically, this was sometimes argued by the same people who were cheering on the claim that there is no deterministic forcing at work in driving temperatures upwards (be it GHG or the sun or whatever else). So much for coherence.
Let’s look at how the global average temperature, CO2 and the sun changed over recent decades:
Temperatures jiggle up and down, but are increasing over the longer term (multiple decades). CO2 has a seasonal cycle due to the ‘breathing’ of the biosphere, but is steadily increasing over the years due to human emissions. The sun shows an 11-year cycle, but no secular increase or decrease over this time period.
Let’s go a bit farther back in time (HadCRU temperatures up to 2008; number of sunspots as a proxy of solar activity; original here):
And looking back at sunspot observations over the past 400 years (original):
So what does this tell us? Of course changes in the sun affect our climate (coherence check: This implies a certain degree of determinism). Low solar activity (e.g. during the Maunder and the Dalton minima) played a role in the so called ‘little ice age’. In the beginning of the 20th century solar activity increased, which contributed to the warming (together with greenhouse gases and a lack of volcanic activity). However, since the solar output (including cosmic rays) remained steady (or even decreased a bit) since the 1950’s, it doesn’t seem very likely that the sun contributed to the recent increase in temperatures since the 1970’s. The little ice age ended (~1850) long before the recent warming started (~1975), so no causal relation there either.
The main reasons that disqualify the sun as being a major culprit in recent global warming are:
• No increase in solar output (or decrease in cosmic rays) over the past 50 years
• Nighttime temperatures increased more than daytime (inconsistent with solar forcing; consistent with GHG forcing)
• Stratospheric cooling (inconsistent with solar forcing; consistent with GHG forcing)
See also Skeptical Science and Lockwood’s recent review paper (the first chapter, describing the context of the ‘controversy’ is well worth reading).