Global Warming Milestones

The Nobel Prize in physics in 2021 went in part to two gentlemen in their 90s: Prof Klaus Hasselmann from Germany and Dr. Syukuro Manabe  from Japan/United States (Figure 1). These two pioneers in climate physics were honored for their ground-breaking scientific contributions which helped to understand and simulate the climate system and its response to increasing greenhouse gas concentrations. The Nobel prize citation emphasizes their paradigm-shifting publications from the 1960s and 1970s.

Figure 1: Prof. Klaus Hasselmann (left) and Dr. Syukuro Manabe (right) received a share of the Nobel prize in physics in 2021 for their ground-breaking contributions to understanding the climate system and global warming.

When I give public presentations, the audience is often surprised to hear, that scientists knew about global warming and its implications already more than 50 years ago. “Why is it that politicians and the industry didn’t act earlier? Why are we still increasing CO2 emissions every year? Why were these scientists honored so late for their contributions?” are common questions that I hear.

To address these questions, allow me to take you on a brief journey from the 1960s, when a handful of scientists developed the first climate computer models, to the late 1980s, when the Intergovernmental Panel on Climate Change (IPCC) was founded; to the 1990s, when Prof. Klaus Hasselmann showed undeniably that humans were to blame for the rising temperatures of the 20th century, to now, when politicians and CEOs meet in luxury resorts to discuss how a planetary climate disaster could be averted, while youth activists demonstrate outside for their right to live on an unaltered planet. How has it come to this point?

Let’s wind back to the mid 1960s when a team of computer programmers at the Geophysical Fluid Dynamics Laboratory in Princeton (USA), spearheaded by atmospheric scientist Dr. Manabe tried to run a new computer code for their atmospheric model on the first generation of commercially available supercomputers (Figure 2). It was almost impossible to crunch the numbers in the way the scientists had originally envisioned, and Dr. Syukuro Manabe and his colleague Richard Wetherald tried to simplify some of the underlying physical assumptions of the mathematical model to make it “digestible” for their supercomputer. The resulting radiative-convective equilibrium model gave one of the first predictions of the effect of CO2 on global temperatures. The key results were published in 1967 and the paper1 includes the visionary statement “According to our estimate, a doubling of the CO2 content in the atmosphere has the effect of raising the temperature of the atmosphere (whose relative humidity is fixed) by about 2oC.” This paper is regarded by many climate scientists as the keystone paper of modern climate science and it is fair to say that it laid the foundation, for everything else that came after it: from the Intergovernmental Panel on Climate Change (IPCC) to net zero carbon, RE100, Conference of the Parties (COP), solar energy subsidies and Greta Thunberg; but I am jumping ahead of myself.

In the 1970s and 1980s, thanks to Dr. Manabe’s new discoveries, other climate modeling centers were launched worldwide; one of them in Hamburg Germany in 1975.  Directed by physicist Klaus Hasselmann, the new Max Planck Institute for Meteorology had to establish itself quickly as the “new kid on the block”. One of the first scientific studies published at the institute in 1976 was a theoretical study2 with the inconspicuous title “Stochastic Climate models, part 1”. The ingenious idea behind the paper was to adopt Einstein’s theory of Brownian motion to explain the interaction between atmosphere and ocean and thus to explain the generation of natural climate variations on a wide range of timescales. This paper was the second paper highlighted by the Nobel Prize committee in 2021. A landmark study in climate science, the “Stochastic Climate model” would lay the foundation for our understanding of natural climate processes, which are generated by the interaction between atmosphere, ocean, and ice.

In the subsequent 15 years the quest began worldwide to detect the anthropogenic greenhouse warming signal in global temperature and weather records. The main challenge was to separate the man-made warming from the naturally occurring shifts and patterns of the climate system, which Prof. Hasselmann’s 1976 study addressed. By applying so-called optimal fingerprinting methods3,4 (a kind of climate-CSI) Prof. Hasselmann and his colleague Prof. Gabriele Hegerl completed this task in 1995. I still remember the TV broadcasts in Germany, which announced that the Max Planck Institute of Meteorology found an unequivocal proof that human greenhouse gas emissions were responsible for most of the increasing temperatures of the 20th Century. It was a sensation, which rocked Europe’s understanding of climate change, and it provided the scientific basis to implement a series of green policies. It also helped wipe away unjustified concerns of policymakers and the industry who were initially worried that major shifts away from fossil fuels may trigger an economic downturn. Hasselmann’s “proof” eventually paved the way for Europe’s leadership in international climate protection. 1995 was also the year when my own journey in climate science began. I joined Prof. Hasselmann’s institute in 1995 as a PhD student. Using one of Germany’s fastest supercomputers at that time (Figure 2), a Cray C90, I was able to study the interaction between the El Niño-Southern Oscillation (ENSO) and man-made climate change5. ENSO is the most powerful naturally-generated interannual climate phenomenon, which influences weather patterns worldwide about every 4-10 years.  Published in 1999, my research motivated several follow-up studies in which scientists subsequently investigated how other natural climate processes respond to man-made greenhouse warming, providing a more complete picture on the effects of climate change on extreme events.

Figure 2: The role of supercomputers in climate science: from Manabe’s CDC 6600 (GFDL, USA) to Hasselmann’s Cray C90 (DKRZ, Germany), Japan’s Earth simulator, South Korea’s Aleph XC50, and the new DKRZ Levante supercomputer. As computing power increased over the past 50 years by almost a factor 10,000,000,000, more earth system model components were included in the simulations.

In the meanwhile (Figure 3), the IPCC had been founded in 1988 by the United Nations with a mandate to assess the science related to climate change. IPCC has since been extremely effective as an intergovernmental body with 195 member states to convey the urgency of the looming climate crisis to policy makers and the general public. In three working groups, the IPCC highlights and discusses key scientific findings in a variety of climate-related areas. Working Group 1 deals with the physical scientific basis of climate change, whereas Working Groups 2 and 3 address socio-economic and ecological impacts of climate change and options to mitigate climate change through policies or technologies, respectively. At the base of the IPCC assessment reports are so called “projections” of the future, which are based on climate computer models, similar in spirit to the ones used by Dr. Manabe in the 1960s, but many orders of magnitude more complex. Nowadays, a typical climate model consists of over one million lines of computer code (typically written in the programming language Fortran), which solve the physical equations of the atmosphere, ocean, land, vegetation, and marine biogeochemistry. The IPCC, which received the Nobel Peace Prize in 2007 (together with former US Vice-President Al Gore), has played a pivotal role in raising global awareness for the climate crisis, and providing detailed information on what to expect if humankind does not abandon its addiction to fossil fuels. It is fair to say that without the pro-bono work hundreds of climate scientists under the IPCC umbrella, none of our current discussions on achieving carbon neutrality on a global or even national level would have taken place.

Figure 3: Timeline of major climate science discoveries from 1901-2021. The shading of the arrow indicates South Korea’s average temperatures (upper figure, licensed from Shutterstock, lower figure adapted from

… and then there are UN’s COP (Conference of the Parties) meetings, the last one in Kairo in late 2022, and the next one to be held by the United Arab Emirates, under the leadership of Sultan Al Jaber, CEO of one of the world’s largest oil companies ADNOC. Some of the previous COP meetings can be considered milestones on our journey to protect the climate system, such as the Paris meeting in 2015, which brought about the famous Paris accord. This agreement aims to “strengthen the global response to climate change by keeping a global temperature rise this century well below 2 degrees Celsius above pre-industrial levels and to pursue efforts to limit the temperature increase even further to 1.5 degrees Celsius”. The Paris agreement also introduced new mechanisms to implement and monitor the pledges for carbon neutrality on a national level, such as the “Nationally Determined Contributions” (NDCs). Not all COP meetings have been as successful as the Paris meeting, and it remains to be seen whether the involvement of the oil industry in COP28 will negatively interfere with the ongoing global efforts to protect the climate for the current and future generations.

Let us recapitulate here. What do we know about the climate system and its response to anthropogenic greenhouse emissions? Why should we even care about climate change?

  • Global warming is real; global mean surface temperatures have risen by 1.2oC since the beginning of the industrial revolution (and by ~2oC in South Korea relative to average 1901-1931, source: CRUTEM5) and most of this warming can be attributed to human fossil fuel burning.
  • Intensified warming (relative to the global mean) occurs on land and in polar regions
  • Climate models predict that lack of climate change mitigation could lead to an additional planetary warming within the next 75 years of about 3oC (as compared to the early 2oC estimate by Dr. Manabe from the 1960s), with catastrophic consequences for global food security, ecosystems, and biodiversity.
  • Additional global warming of more than 0.6oC above present-day levels could trigger an irreversible melting of the Greenland and West-Antarctic ice-sheets with longterm effects on global sea level, causing coastal inundations and human displacement
  • In a warmer climate, we expect dry regions to become drier in future, and wet regions wetter, which will impact agriculture and food production.
  • Moreover, rainfall events on land, irrespective of where they will occur, will become more intense, which is likely to increase the chances for flooding, crop losses and infrastructure damage (Figure 4)
  • Anthropogenic CO2 emissions have been partly taken up by the oceans, where they have already caused unprecedented levels of ocean acidification (a reaction that negatively impacts calcifying organisms in the ocean, such as corals). Future ocean acidification would push ecosystems and marine food webs way outside of what they have experienced within the past 55 million years. At this stage it is unclear, how the ocean’s ecosystems would reorganize under such stress.
Figure 4: Amplification factor for  extreme rainfall events over next 75 years if no efforts are made to cut down CO2 emissions. Projections are from the large ensemble simulations6 of the IBS Center for Climate Physics, run on supercomputer Aleph (Figure 2) using greenhouse gas emissions scenario SSP3-7.0. The figure illustrates how often a rainfall event that now occurs only once every 10 years will occur in a ten-year segment in 2090-2100.

These are just some of the impacts caused by fossil fuel burning and given this bleak outlook for the future climate, it is understandable that the global youth, which would bear the brunt of these anticipated changes, is anxious to see climate action, rather than yet another high-level UN meeting fail, because of conflicts of interest or simply lack of ambition.

So, what can we do about it? Should we be optimistic? Is there a way forward? From a scientific point of view, it is simple and clear what needs to be done: we need to cut down CO2 emissions as soon as possible and reach net zero carbon globally within the next two to three decades. Further delays and wavering mean that our children’s generation will suffer even more in the decades to come – exactly Greta Thunberg’s point. How carbon neutrality is best implemented now depends on the individual countries and their current infrastructure and available resources. Decarbonizing entire economies requires us to first put a price on carbon to account for investments and damages. This was recognized already in the 1970s and 1990s by William D. Nordhaus, who eventually received the Nobel Prize in economics in 2018 for developing the first integrated climate-economy models7,8. Honest carbon pricing can be best implemented by introducing carbon tax and dividend systems, which disincentivize the use of fossil fuels and reward the use of carbon-free products and technologies. From there, countries need to identify their optimal renewable energy portfolios that would lead them quickly to a net zero emissions.

Figure 5: Longterm average of potential photovoltaic electricity production (kWh/kWp) for Germany (left) and the Korean Peninsula for a free-standing power plant with optimum tilt.

For South Korea, which in 2021 obtained only less than 10% of its energy (2022 data have not been released yet officially) from renewable energy resources, the lowest hanging fruit would be massive investments in solar power generation. South Korea is — contrary to commonly-held believes — a country which would be optimally suited to harvest solar power. The numbers speak for themselves: Busan has 46% more sunshine hours per year compared to cities like Frankfurt or Hamburg in Germany, Yet even rainy Germany generates 8.5% of its electricity from photovoltaics (in 2021), whereas South Korea trails at ~5% (in 2021). Moreover, South Korea is at a lower latitude than Germany, which makes sunshine much more intense and increases the photovoltaic power potential (Figure 5).

But we must be honest, carbon neutrality cannot be achieved by technologies alone. It is worth remembering that South Korea is still one the countries in the world with the highest per capita CO2 emissions.  Since it is in the end our personal lifestyle that is causing global warming, cutting down CO2 emissions requires tangible sacrifices from all of us. Having a reusable cup for our morning coffee won’t suffice. The actions that need to be taken to achieve carbon neutrality will impact our lifestyle and temporarily also our convenience. Achieving carbon neutrality is not about cutting corners, green-washing, procrastinating or maintaining the illusion that we can completely engineer ourselves out of this crisis one day. It is about decarbonizing our daily lives today, implementing and supporting circular economies, divesting from fossil fuels, switching to renewables and saving energy.

Axel Timmermann, Distinguished Professor

Director, IBS Center for Climate Physics

Pusan National University, Busan

South Korea


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  2. Hasselmann, K. Stochastic Climate Models. 1. Theory. Tellus 28, 473-485 (1976).
  3. Hasselmann, K. OPTIMAL FINGERPRINTS FOR THE DETECTION OF TIME-DEPENDENT CLIMATE-CHANGE. Journal of Climate 6, 1957-1971 (1993).
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