The increase in emissions of greenhouse gases into the atmosphere is only one of several serious global threats to our continued existence on Earth, and their reduction is at the core of international agreements like the Paris Agreement and the UN Sustainable Development Goals. Together with Ukrainian colleagues, IIASA researchers took a novel approach to further the understanding of the planetary burden and its dynamics caused by emissions from human activity.
In order to better understand the burden of human activity and its dynamics on the planet, specifically as it relates to the effects of the continued increase in greenhouse gas (GHG) emissions and global warming, researchers from IIASA and the Lviv Polytechnic National University, Ukraine, approached the problem from a "stress–strain"-perspective. Going beyond conventional textbook knowledge, the team introduced three parameters that characterize the system into their model: delay time, memory, and persistence.
The ability of a system to build up memory can be understood as its ability to respond within its natural mechanisms or, if the build-up of memory is limited, as a measure for future global system failures. This ability declines considerably with memory reaching high levels of exploitation. In this regard, the authors note that approximately 60% of Earth's memory had already been exploited prior to 1959. Persistency refers to how locked in planetary processes can become, making it progressively more difficult to relax the system.
The three parameters depend, all things being equal, solely on the Earth system's characteristic rheological (viscoelastic) behavior and allow deeper and more novel insights into that system. The researchers viewed the emission of manmade GHGs into the atmosphere, notably carbon dioxide (CO2), as a stressor on the Earth-system and surveyed the condition of Earth in terms of stress–strain units. This perspective goes beyond the global carbon mass balance perspective typically applied by the carbon community, which is widely referred to as the gold standard in assessing whether Earth will remain hospitable for life in the future.
The researchers began with the stress caused by the CO2 emissions from fossil fuel burning and land use between 1959 and 2015, confirming that, from the standpoint of a global observer, the CO2 concentration in the atmosphere has increased rather quickly over this period. At the same time, the atmosphere has been reported to have warmed and expanded by approximately 15 to 20 meters in the troposphere per decade since 1990, while part of the carbon has been locked away (rather slowly) in land and the oceans. Together, the expansion of the atmosphere and the uptake of carbon in so-called carbon sinks, are referred to as the overall strain response of the atmosphere–land and ocean carbon system.
According to the authors, it is not clear how reversible and divergent the slower process (uptake of carbon by sinks) is in relation to the faster one (expansion of the atmosphere). What is known, however, is that the former process remembers the influence of the latter, which runs ahead. This led the researchers to ask three questions, namely: Can this global-scale memory ̶ Earth's memory ̶ be quantified? What is the degree of depletion? And, does Earth's memory allow its persistence to be quantified, speculating that the two are not independent of each other?
To answer these questions, the authors let "memory" extend back in time to 1850, assuming zero stress from human activities before that date. They found that since 1850, the atmosphere, land, and ocean system has been trapped progressively in terms of persistence (i.e., it will become progressively more difficult to relax the system), while its ability to build up memory has been reduced. Concomitantly with the exploitation of memory, the study found that the limiting value that persistence aims at, increased by approximately a factor of two to three since 1850 and can be expected to increase further if the release of CO2 emissions continues globally as before.
"The atmosphere, land, and ocean carbon system is much more fragile than is widely believed. Based on the stress–strain insights from our study, we expect that the atmosphere, land, and ocean carbon system could be forced outside its natural regime well before 2050 if the current trend in emissions is not reversed immediately and sustainably. These insights are independent from any external target values such as temperature targets justified by means of global change research, and suggest that the time window for counter measures, including mitigation and adaptation, is much shorter than we think," says study lead author Matthias Jonas, a senior researcher in the IIASA Exploratory Modeling of Human-Natural Systems Research Group.
The authors note that although the focus of their study is on the atmosphere, land, and ocean carbon system, the stress–strain approach they followed should not be considered an appendix to a mass-balance-based carbon cycle model. Instead, it should be seen as a self-standing model belonging to the suite of reduced but still insightful models that offer great benefits in safeguarding complex three-dimensional climate and global change models, among which a stress–strain model is currently not available.
The study has just been published in the journal Earth System Dynamics.