My name is Allegra LeGrande. And, I am a climate scientist at the NASA Goddard Institute for Space Studies (GISS) and Columbia University Center for Climate Systems Research in New York City. Investigating climates of the past have been my passionate research topic for 20 years now.
At GISS, I am working on this project with two researchers, Kostas Tsigaridis – a specialist in aerosols and a research scientist like me, and Ram Singh, a post-doctoral research scholar who came to GISS especially to work on this project.
I started writing this blog post in April of 2020. I live in New York City, several miles north of Columbia University’s campus. As I write, their are 1000’s New Yorkers who have lost their lives to the SARS-CoV-2 virus (most in hospitals, but many at home). The soccer stadium next to my home has a pneumatic dome being converted into a 250-bed field hospital. It is a challenging time to do climate research as first responders and essential workers put their own lives at risk on a daily basis. Now, in July 2020, just as NYC seems to have gotten our crisis under control (losing upwards of 25,000 people along the way), the rest of the country is suffering.
I am writing about my research because the privilege afforded to me by their sacrifice. Thank you to all first responders and delivery and retail and hospital workers. Thank you.
Figure 1. Hyde3.1 crops+pastures data set for 3000 years ago; figure from Kaplan et al 2011.
How do people change with time? Do we co-exist with our environment? Does it influence us? Do we influence the it back? Are humans reacting to changes or partners in exacting change?
These questions are central to the NSF Climate and Natural History research program.
Certainly, the idea that TODAY people are changing the climate in very real ways is not new. At NASA GISS, we have been keeping up with temperature trends over the last 150 years. Much of this trend can be attributed to humans releasing excess amounts of greenhouse gases like CO2. The recent COVID19 crisis has shown how much human activity was altering the air we live in by stay-at-home measures decreasing people’s emissions of environment altering gases.
But what about in the past – really far back in human history. Were people just as influenced by the climate? (Yes!) And, were people somehow able to influence climate back again? Maybe – the ‘early anthropocene’ subject area is highly uncertain.
It could have been, in very regional settings in places where human density was high that humans could have had a regional impact on climate. Figure 1 shows some of the rare places in the world where there were enough humans to have substantive change on the land surface. The Nile River Valley is one of those places. During the flood season, this human alteration of the landscape would have been even greater, with vast swaths of land inundated — Figure 2.
Figure 2. Flooding of the Nile River expands the human-altered landscape by 1000s of km2. Metwaly et al (2005).
This forms the basis of the YALE-COLUMBIA-NASA GISS Yale Nile Initiative research program. We are researching whether human alterations of the Nile River flood basin influenced the climate response.
How do we simulate climates of the past?
Figure 3. Pre-Industrial versus Non- Anthropogenic aerosols.
As a first step, we have to remove all the forcing of people. Even our baseline ‘pre-industrial’ simulations for the climate model have some impact of people. First, we remove all of the anthropogenic land use. Then we set the greenhouse gases to the concentration to that at year 1850. This is called the pre-industrial simulation.
The pre-industrial simulation for our climate model, the NASA GISS ModelE2.1 is described in 2020 here by Max Kelley and the rest of the GISS Modeling team.
As a second step, we remove all of the anthropogenic aerosols from the climate system, too – even those that existed in 1850. This is called the non-anthropogenic aerosol simulation. Figure 3 shows that the entire world’s temperatures weren’t so different. But, there were regional patterns of change.
Figure 4. Insolation differences for 500 BC compared to present day. (Based on data by Berger and Loutre, 1991.) Figure made using Panoply.
Finally, we are ready to set the stage to simulate the earth of 2,500 years ago.
Milutin Milankovic proposed that small variations in the earth’s orbit around the sun could alter long-term climate. These variations imply that there would have been attendant changes into the distribution of sunlight across the earth’s surface. Incoming SOLar radiation (insolation, for short) – to the earth’s surface was a little bit different.
André Berger and Marie-France Loutre (1991) sorted out the equations that show exactly how much isolation changes through the year – find this in Figure 2.
Compared to present day, the 500BC Northern Hemisphere had more insolation in summertime (red colors), while the Southern Hemisphere had more insolation in its spring. This yielded changes in the patterns of temperature and precipitation.
Figure 5. Pre-Industrial versus 2.5 kya (orbital and greenhouse gas changes).
Another vital change to the climate system of 2500 years ago is the decrease of atmospheric methane (Figure 2). No one completely understands why methane was lower in the mid-Holocene. Many think it was a balance of the destructive (oxidative) capacity of the atmosphere reducing the ability of methane to exist and a decrease in production. The decrease in production might have been driven by alterations in Northern Hemisphere seasonality, changes in the biota of high latitude peatlands, (or the depth of the water table). Whatever the cause, it made the climate a little different from today.
The decreases in methane and other greenhouse gases make the planet cooler on the whole.
Overall, these changes in insolation and orbit 2500 years ago meant that the tropics were somewhat cooler on the whole, and that seasonality was larger – warmer summers and cooler winters.
This change would have had profound implications for peoples in the African continent since the northmost extent of the rain belt – the Inter-Tropical Convergence Zone – would have been further north than it is today. Earlier than our focus period – 5,000 to 6,000 years ago – this widescale alteration and ‘wetting’ of the continent is known as the African Humid Period.
Numerous paleoclimate studies and even direct indicators of paleo-human populations hint at how different the climate was compared to today.
Figure 6. Anomalous daily (mm) rainfall in the Nile River catchment basin and larger region 2500 years ago versus the pre-industrial period in GISS-E2.1-R.
This enhanced precipitation would have stretched the width of the African continent, including the Ethiopian Highlands, the headwaters for the Nile River.
Figure 6 shows the impact of the orbital and greenhouse forcing changes alone on precipitation. Vegetation changes would have augmented and reinforced this greener Africa. In our next blog post, we work through the ways the vegetation feedback would have reinforced this African greening.
Both of these features will show themselves to be more and more important as we consider the impact of strato-volcanic events in this region.
Author: Allegra N. LeGrande, Co-PI (Columbia University, NASA GISS)