Welcome to YouChoose, a blog about individual climate action.
This past summer, when talking about climate change with friends and family I often heard the argument that changes have always happened and the Sun or other natural causes are responsible for them. Invariably, past glaciations and the Little Ice Age are mentioned. I was curious to understand if this view was common and so I looked at recent statistics on the subject. That is when I found out that about 46% of the population believe that climate change is due to human activity and the remaining 54% of the population think that there is no evidence of change in temperature (14%) or believe that there is a change but is due to natural causes (26%) or is not sure (14%). I have therefore decided to start a series of short posts to address these perceptions hoping to clarify some doubts and misconceptions.
As a premise, I hold a PhD in physics and work and publish in the climate field. I am not a climate modeler but work with large datasets of observations of climate variables from satellite and ground observations and more importantly, have access to relevant journals. The topic of climate change is very complex and encompasses numerous specialized disciplines. In addition to the intrinsic complexity of the field, there is a lot of often deliberately confusing information published in the news. For all these reasons I am starting here a series of short posts that discuss the evidence leading scientists to conclude that current observed global temperatures are not a response to natural forcing, but are largely due to human activities. I deliberately keep each post short and limited only to one topic. In this first post we review how the Earth’s climate has changed during the course of millions of years. This knowledge gives us a broad perspective on how the short time span during which our human species has occupied the Earth compares to the past.
Our Earth’s past climate
To place climate change in a broader perspective it is important to study the climate of the past, when human activities did not exist. Knowledge of how the Earth’s climate has reacted to changes in the past helps us understand better what has been happening in the last 100 years against a background of natural climate variability and it also guides scientists to forecast future climate scenarios. The reconstruction of past climates from paleo data is a difficult field, and many uncertainties affect the data. The presence of uncertainty is an intrinsic part of the scientific process, however when analyzed and treated properly, uncertainties do not invalidate the results, rather they make them more reliable.
In past times, just as today, the Earth’s climate has changed periodically due to a combination of factors including orbital changes, changes in greenhouse gases, and changes in the extension of the ice caps. For example, for many million years, global surface temperatures were several degrees higher than they are now. Large scale changes to the climate are generally triggered by astronomical changes, usually related to the Earth’s orbital path, and are then amplified by melting (or freezing) of the glaciers and changes in greenhouse gases. We’ll review natural triggers to climate in detail in a next post. We currently live in an epoch called Holocene that has been one of relative climate stability and is called an interglacial age. During the Holocene and especially in the recent past until preindustrial times, the Earth average temperature has been quite stable, allowing human activities to develop relatively undisturbed.
A few million years ago…
Through the ages our Earth has experienced many changes. Fig. 1, from Hansen et al., 2013, shows a reconstruction of global surface temperature in the last 65 million years. The figure is freely accessible on the internet along with the publication. You can read at the end of this post how deep ocean sediments are used to reconstruct global surface temperature to track paleoclimate. The top panel shows the Earth surface temperature starting 65 million years ago until the current Holocene Epoch. The middle and bottom panels zoom in the last 5 million years and the last 800,000 years.
What does this figure tell us?
At the beginning of the Cenozoic, the Earth was much warmer than today, more than 10 degrees warmer. During those times sea levels were much higher, ice caps were smaller, and CO2 levels were about 3 times what they are today. From the beginning of the Cenozoic Era the Earth’s temperature has been decreasing, becoming about 12 degrees cooler on average in the course of 50 million years but with oscillations of warming and cooling periods visible in the middle panel. From the middle panel we can discern that the amplitude of these oscillations is 4-7 degrees.
Zooming into the last 800,000 years, shown in the last panel, we can have a better look at how these oscillations look like. In the past 800,000 years there have been 8-9 cold/warm cycles alternating glacial and interglacial ages, with the last ice age, marked by an arrow in the figure, happening approximately 12,000 years ago. These cycles of glacial-interglacial periods last about 100,000 years during which the temperature changes 4-7 degrees from minimum to maximum, oscillating between 8 and 15 degrees Celsius. The last inter glacial period that started 12,000 years ago is the Holocene and is the time during which we humans have lived. Two prior inter glacial periods were warmer than the Holocene: the Eemian (~ 130,000 years ago) and the Holsteinian (~ 400,000 years ago). In both periods the sea level height was at least 3-5 meters (9-15 feet) higher than today (see for example Cuffey and Marshall, 2000).
How does all this this relate to climate change?
Scientists use past climates to understand what caused changes in order to assess if current observed trends can be attributed to natural causes. They also use past temperatures to compare to current observations and especially to the current rate of temperature changes.
Take home points from this first post:
- The Earth climate has changed during the past 60 million years, generally cooling, but also going through oscillations between glacial and inter glacial periods.
- These oscillations took roughly 100,000 years to complete and during the 100,000 years it took to complete a cycle the Earth temperature changed 4-7 degrees from minimum to maximum.
- We currently live in an interglacial period called Holocene
- During the last 2 interglacial periods before the Holocene (about 400,000 and 130,000 years ago) temperatures probably reached at least 1 degree higher than the preindustrial temperature.
In the next post we’ll zoom into the Holocene (last 12,000 years) and then into the last few thousand years and we’ll compare those data with the more recent global temperature measurements.
References:
- Hansen J., Sato M., Russell G. and Kharecha P., 2013: Climate sensitivity, sea level and atmospheric carbon dioxide, Phil. Trans. R. Soc. A. 371:20120294 http://doi.org/10.1098/rsta.2012.0294
- Cuffey, K., Marshall, S., 2000: Substantial contribution to sea-level rise during the last interglacial from the Greenland ice sheet. Nature, 404, 591–594. https://doi.org/10.1038/35007053
Appendix
How are past global surface temperatures reconstructed?
Temperature back in time are reconstructed from oceanic sediments with the help of an isotope of oxygen (Oxygen-18) found in deep ocean sediments. This standard methodology uses the fact that most of the Oxygen has 8 neutron (Oxygen-16), but a small percentage of oxygen molecules occurs naturally with 10 neutrons (Oxygen-18). Oxygen-16 evaporates more promptly and enters in the Earth hydrologic cycle through precipitation and freezing. In a warmer climate the Oceans, and therefore its inhabitants, contains more of the Oxygen-16 isotope, in colder climate they contain more of the Oxygen-18. Moreover there is a well-defined relationship between the ratio of the 2 isotopes and the change in air temperature that allows to translate the sediments found into temperature once calibrated with a well-known reference. [https://pages.uoregon.edu/rdorsey/geo334/O-isotopes.html]
Additional web site
If you are interested in detailed information or in-dept discussions on recently published work I recommend this web site, maintained by climate scientists: https://www.realclimate.org
The NASA climate page explains the current state of the knowledge on climate at: https://climate.nasa.gov/
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