One thing that scientists and conservative Christians may agree on is that there was a garden of Eden six thousand years ago; however, they may not agree on it's location nor its size. The garden I speak of is what is now the Sahara Desert. Ray Pierrehumbert describes the mid-holocene climatic optimum as a time in which forests extended farther north in Europe and rivers flowed in the Sahara (Principles of Planetary Climate, Chapter 1). The wet Sahara was probably a climate response to Earth's orbital precession, Earth's wobbling top-like motion that determines how the sun's energy is distributed globally.
Six thousand years ago seems to have been the warmest and wettest period of the interglacial era that began twelve thousand years ago when the last Ice Age ended. I would like to ask two questions. First, if the increase of carbon dioxide in the atmosphere is allowed to continue, shall we arrive at a climate similar to the climate of six thousand years ago when the Sahara was wet? Second, if we could choose between the climate of today with a dry Sahara and the climate of six thousand years ago with a wet Sahara, should we prefer the climate of today? My second heresy answers yes to the first question and no to the second. It says that the warm climate of six thousand years ago with the wet Sahara is to be preferred, and that increasing carbon dioxide in the atmosphere may help to bring it back. I am not saying that this heresy is true. I am only saying that it will not do us any harm to think about it. (Dyson 2007, Heretical Thoughts about Science and Society)The statements in this passage struck me as odd. I've perused many Holocene climate reconstructions and I didn't recall any prominent global spikes in temperature from this period, but my knowledge is incomplete. As a check on my memory, I consulted some graphs of ice core data plotted for Greenland and Antarctica and confirmed that nothing exceptional affected Greenland and Antarctica:
The chart above is from a program I created 6 years ago to help me better understand orbital correlations with climate change. I displayed temperature data from Antarctic and Greenland ice cores and plotted it against orbital changes. I zoomed in to the Holocene and marked the time periods of 7000 years ago, 6000, and 5200 when the wet Sahara abruptly ended (Principles of Planetary Climate). The white box is drawn around graphs of peak summer sunlight for five latitudes of the globe. The temperature data do not suggest that the mid-Holocene was a globally warming world, so, the best I could say is that the wet Sahara was a regional phenomenon, some places being warmer and wetter, other places, not. In fact, NOAA, supports this:
...the mid-Holocene, roughly 6,000 years ago, was generally warmer than today, but only in summer and only in the northern hemisphere. More over, we clearly know the cause of this natural warming, and know without doubt that this proven "astronomical" climate forcing mechanism cannot be responsible for the warming over the last 100 years.
The Mid-Holocene "Warm Period", NOAAAlso from my graph above, you can see an interesting change caused by Earth's presession: about 7000 years ago the northern and southern hemispheres started receiving equal sunlight in their summers. This is shown by the graphs of northern hemisphere summer (green lines) crossing the graphs of the southern hemisphere summer (yellow lines). Throughout all this orbital change, the total energy received for the whole planet (white line) is nearly flat, wavering by less than 1 watt per meter. So, the condition creating the climate 6000 years ago is the equal distribution of sunlight, whereas the condition humanity is creating is a global average increase of nearly 3 watts per meter and growing (EPA). Two different recipes suggest two different results.
If you check out NOAA, you may want to peruse their list of paleoclimate references, many of which pre-date Dyson's 2007 essay.
So at this point I'm wondering what evidence Dyson had to suggest a global increase in CO2 with a global increase in global temperature is a path to a wet Sahara with no net losses?
By losses, I'm referring to climate assets that humanity relies on: for example, the Asian monsoon. The Asian monsoon provides and India and China with the water for their rivers and agriculture. The monsoon also builds up the Himalayan snow pack, which are the summer and long term water reservoirs of the region.
I recall from my reading that the monsoon system was not always reliable. It is one of the climate assets that we could lose under a globally or even regionally changing climate. A little hunting produced the article I recalled from 2008:
Millennial- and orbital-scale changes in the East Asian monsoon over the past 224,000 years, Nature 451, 1090-1093 (28 February 2008)So what is a lay climate enthusiast to take from this article? One, the East Asian monsoon is a topic of interest, so it should not be overlooked in evaluating the pros and cons of disturbing Earth's climate. Two, the same orbital dynamics that created a wet Sahara also influence the strength of the Asian Monsoon. Three, the Asian monsoon has weakened and intensified in response to broader climate changes.
High-resolution speleothem records from China have provided insights into the factors that control the strength of the East Asian monsoon. Our understanding of these factors remains incomplete, however, owing to gaps in the record of monsoon history over the past two interglacial–glacial cycles. In particular, missing sections have hampered our ability to test ideas about orbital-scale controls on the monsoon, the causes of millennial-scale events and relationships between changes in the monsoon and climate in other regions. Here we present an absolute-dated oxygen isotope record from Sanbao cave, central China, that completes a Chinese-cave-based record of the strength of the East Asian monsoon that covers the past 224,000 years. The record is dominated by 23,000-year-long cycles that are synchronous within dating errors with summer insolation at 65° N, supporting the idea that tropical/subtropical monsoons respond dominantly and directly to changes in Northern Hemisphere summer insolation on orbital timescales. The cycles are punctuated by millennial-scale strong-summer-monsoon events (Chinese interstadials1), and the new record allows us to identify the complete series of these events over the past two interglacial–glacial cycles. Their duration decreases and their frequency increases during glacial build-up in both the last and penultimate glacial periods, indicating that ice sheet size affects their character and pacing. The ages of the events are exceptionally well constrained and may thus serve as benchmarks for correlating and calibrating climate records.
- So, should we prefer to have a climate of today plus a wet Sahara? Yes. but Dyson never established that we ever did have such a climate. We only know that summers were warmer for the northern hemisphere and that the Sahara was wetter.
- Is global warming a path to this garden of Eden climate? There is no certainty that the regional distribution of sunlight determined by orbital factors can be mimicked by spiking the atmosphere with a doubling of CO2.
- If we did think adding CO2 is a recipe for a wet Sahara (with no unpleasant trade offs) at what point do we stop adding CO2? Yesterday? 1990? Today? 2030?
Question 1 is a topic for paleoclimate research, which may have the answer that I'm not aware of.
Questions 2 and 3 are the domain of climate modelling.
In a brief hunt for more information, I found this:
Recent intensification of tropical climate variability in the Indian Ocean Nature Geoscience 1, 849 - 853 (2008)
The interplay of the El Niño Southern Oscillation, Asian monsoon and Indian Ocean Dipole (IOD) drives climatic extremes in and around the Indian Ocean. Historical and proxy records reveal changes in the behaviour of the El Niño Southern Oscillation and the Asian monsoon over recent decades. However, reliable instrumental records of the IOD cover only the past 50 years, and there is no consensus on long-term variability of the IOD or its possible response to greenhouse gas forcing. Here we use a suite of coral oxygen-isotope records to reconstruct a basin-wide index of IOD behaviour since AD 1846. Our record reveals an increase in the frequency and strength of IOD events during the twentieth century, which is associated with enhanced seasonal upwelling in the eastern Indian Ocean. Although the El Niño Southern Oscillation has historically influenced the variability of both the IOD and the Asian monsoon, we find that the recent intensification of the IOD coincides with the development of direct, positive IOD–monsoon feedbacks. We suggest that projected greenhouse warming may lead to a redistribution of rainfall across the Indian Ocean and a growing interdependence between the IOD and Asian monsoon precipitation variability.I consider this a partial answer to whether our warming is producing a desirable Holocene climatic optimum. Our current global warming trend appears to be disturbing the Asian monsoon, though it is not clear to me which direction we're pushing it.
For me to be optimistic about a return of the Mid-Holocene Climatic Optimum, I need to know the following:
- Was 6000 years ago a global net improvement for a planet supporting 7-9 billion people, most of which do not live near the Sahara?
- Can climate models produce the climate of 6000 years ago?
- Can we model a scenario in which global warming produces a better climate, and if so, when do we apply the brakes?
- And last, assuming we can model a path to a better climate, when do we expect it?. Over night? in 30 years? 100 years? 1000 years? What happens in between?
I do not believe there is a simple prescription that would bring back a wet Sahara. However, I do intend to do more research into what is known about this time.
-------------------- My notes for further research: -----------------------
Asynchronous Holocene optimum of the East Asian monsoon
Quaternary Science Reviews
Volume 19, Issue 8, April 2000, Pages 743–762
Zhisheng Ana, Stephen C. Portera, b, , , John E. Kutzbachc, Wu Xihaod, Wang Suminge, Liu Xiaodongf, Li Xiaoqianga, Zhou Weijiana
The spatial and temporal distribution of summer monsoon precipitation (or effective moisture) during the Holocene has been reconstructed on the basis of geological data, including lake levels, pollen profiles, and loess/paleosol records. In addition, the summer (July) precipitation increment, effective precipitation, and monsoon strength index have been obtained from numerical modeling experiments. Both geological data and numerical modeling indicate that the Holocene optimum, as defined by peak East Asian summer monsoon precipitation, was asynchronous in central and eastern China, reaching a maximum at different times in different regions, e.g., ca. 10,000–8000 yr ago in northeastern China, 10,000–7000 yr ago in north-central and northern east-central China, ca. 7000–5000 yr ago in the middle and lower reaches of the Yangtze River, and ca. 3000 yr ago in southern China. In southwestern China the maximum appeared ca. 11,000 yr ago, but probably was related to the maximum landward extension of the Indian summer monsoon. The regional shift in the maximum precipitation belt from northwest to southeast over the past 10,000 yr is interpreted as a response to changing seasonality related to orbital forcing of the climate. The southeastward shift of the East Asian summer monsoon maximum is consistent with the progressive weakening of the summer monsoon as the summer solar radiation anomaly decreased progressively through the Holocene and the East Asian monsoon index declined, while the early maximum in southwestern China matches the maximum of the Indian monsoon index.
Synchronous or asynchronous Holocene Indian and East Asian summer monsoon evolution: A synthesis on Holocene Asian summer monsoon simulations, records and modern monsoon indicesHolocene climate records obtained from the Asian summer monsoon domain suggest a regionally-delineated response to changing summer monsoon. The interaction between the East Asian summer monsoon (EASM) and the Indian summer monsoon (ISM), two subsystems of the Asian summer monsoon, has been considered as a factor that explains those inconsistent Holocene climate records. However, this assumption is not valid when the relationship between the two subsystems is not clear. This paper presents a literature review regarding climate simulation of the Asian summer monsoon for testing the long-term relationship. The absolutely-dated Holocene speleothem records in the EASM domain and the ISM domain were compared to verify the simulation results. In addition, a unified monsoon index, which has a unified solid dynamic basis and is appropriate for different monsoon regions, was used in order to identify the modern relationship between the two subsystems. The speleothem records show more synchronous than asynchronous on the Holocene millennial-scale monsoon evolution, furthermore the two subsystems respond to the Younger Dryas (YD) and 8.2 ka events in a similar way. However, these monsoon simulations roughly suggest that the two subsystems respond to Holocene climate change in different ways. While the simulations were mostly performed in a certain period of the Holocene, the speleothem records provided a relatively continuous Asian summer monsoon history. Therefore, time scales could affect the comparison between simulations and speleothem records. Then, we further discussed the interaction between the Asian monsoon subsystems according to simulations and modern monsoon indices. Overall, the relationship between the two subsystems is more complicated than synchronous or asynchronous, which is a dynamic relationship and related to the atmosphere–land–ocean–vegetation interaction. In addition, the relationship can vary over different time scales, and the links between time scales should be paid more attention to. Besides, the interaction between the westerly winds and the Asian summer monsoon in the mid-latitudes of East Asia will profoundly affect those areas in response to Holocene climate change. It is recommended that further research should be emphasized in dynamic mechanisms between the Asian summer monsoon subsystems and between the Asian monsoon and the westerly winds.
Global and Planetary Change
Yu Li ⁎, Nai'ang Wang, Xuehua Zhou, Chengqi Zhang, Yue Wang
Global and Planetary Change
Yu Li ⁎, Nai'ang Wang, Xuehua Zhou, Chengqi Zhang, Yue Wang
Update: The following illustration is being added on 2 November 2015 to assist an explanation in the comments: