Climate has varied before – many times. And all were caused by natural forces.
Climate as a rule changed due to variations in insolation – solar forcing – or due to redistribution of heat by changes in ocean currents.
The sun could change in intensity, like the “Little ice age”. During this period there was very little sunspot activity, and less energy from the sun arrived on our planet. At other times the Earth’s orbit around the sun could vary, increasing or decreasing the distance between our planet and the sun. At other times the orbit could be more elliptical, or the polar axis on our planet could increase or decrease its angle. With a decreasing angle, the land areas on the Northern hemisphere would receive more sunlight, and thus become warmer. These astronomical effects are well accounted for and are called the Milankovich cycles.
So if you hear that the climate on Mars has slightly warmed without much human activity there, that might be true. At the same time Saturn has cooled. This has nothing to do with our situation on Earth. This is simply Milankovich cycles on other planets.
Earth’s climate has been varying like a roller coaster. The IPCC even has a separate chapter on the climate in previous times called Paleoclimatology (IPCC 2007) ||Paleoclimatology IPCC 2013 || IPCC 2013 WG 1 chapter 5 |
However, what happens now, is no natural climate variation, it is a real climate change caused by human activities, such as burning of enormous amounts of fossil fuels, the clearing of rain forests, land use change, cement production and new forms of agriculture.
There is no special warming Milankovich effect like we had in the Holocen optimum some 5000-8000 years ago; there are no big non-eruptive volcanoes pumping greenhouse gases into the air. There is no other explanation for the present global warming than human activity. We can already observe how the Earth’s climate and with it the world’s ecosystems are changing at an accelerating pace.
The warm holocen optimum
In the heated global warming and climate change debate we often hear about the warm period 5000-8000 years ago, the holocen optimum period. That warming was most likely due to the Milankovich effect with a decreased angle on the polar axis and a change in the Earth’s orbit. It seems the summers were warmer than usual, not the winters. There was no warming in the Southern hemisphere. In other words: no global warming took place at that time, and the holocen optimum cannot be compared to our current situation: different time, different causes, different and regional impact. See The mid holocene “warm period”.
The last 50 years we cannot attribute global warming to the sun. The solar forcing – the insolation – has not increased the last decades. The insolation has actually decreased. With no other forcing factors, earth should probably have experienced a colder period for the time being.
Long term temperatures do not correspond to solar activity, only to CO2 levels, although short-term fluctuations do appear due to solar activity and El Nino cycles. The chart below compares CO2 levels and global temperatures. There are short-term fluctuations due to solar activity and El Nino cycles, but there is a clear correlation with CO2 level in the long term.
Two other factors indicate that it is greenhouse gases that cause the warming rather than changes in solar activity.
First, while lower atmosphere temperatures have gone up, upper atmosphere temperatures have gone down. If the increase in temperature were due to more solar energy, all levels of the atmosphere would be getting hotter. However, the theory that greenhouse gases are trapping heat in the lower atmosphere also explains the lowering of temperature in the upper atmosphere.
Second, average nighttime temperatures have increased more than average daytime temperatures. If increased solar energy were causing the heating, the days would be getting hotter faster than the nights. However, since the nights are not cooling down as fast and therefore getting hotter. this only makes sense if greenhouse gases are trapping heat in the atmosphere and not letting the night air cool as fast.
- Why natural cycles only play small role in rate of global warming
- Sir David Attenborough: The Truth About Climate Change‘
- Science: Not Much Warming Under the Sun
- Solar Constant: Construction of a Composite Total Solar Irradiance (TSI) Time Series from 1978 to the Present
- The NASA data conspiracy theory and the cold sun
- NOAA temperature record updates and the ‘hiatus’
- Royal Society: What role has the Sun played in climate change in recent decades?
- Climate (National Centre for Atmospheric Research)
- NOAA/NCDC – Climate Change: Incoming Sunlight Are humans responsible for global warming?
- Nature: climate change
- What does past climate change tell us about global warming?
- The Holocene
- Geologic temperature record (Wikipedia)
- Global warming hiatus explained and it’s not good news
- Ten Clear Indicators Our Climate is Changing | Climate Reality
- Teaching Essential Principle 1:
The Sun is the Primary Source of Energy for Earth’s Climate System
- How Does the Sun Affect Our climate?
- Correlation of global temperature with solar activity
- Astronomical Theory of Climate Change (Milankovich cycles)
- That Global Warming Hiatus? It Never Happened. Two New Studies Explain Why. (2018)