Since the start of the industrial era around 1800, emissions of GHGs through human activities have grown nearly exponentially. The resulting changes in atmospheric composition and climate provide powerful signals that human activities are modifying the functioning of the Earth system itself.
Climate change now has the attention of the world, both as a profound challenge in its own right and also as a symbol of the entire set of human-induced pressures on planet Earth. At Copenhagen in 2009, the international community said, “we underline that climate change is one of the greatest challenges of our time … we agree that deep cuts in global emissions are required … to hold the increase in global temperature below 2°C.” Thus, a 2°C target now has international support. Whether or not 2°C is a sufficiently strong target is still debated; however, it is a start.
Nevertheless, among many climate scientists and in many parts of the broader community, there is an increasing feeling of desperation: a sense that it may now be too late to hold global warming above pre-industrial temperatures to 2°C, let alone to tougher targets such as 1.5°C or less. Among the causes of this desperation is the apparently inexorable growth in anthropogenic GHG emissions, despite over two decades of intense international effort.
As calls for action have become more urgent, the growth in GHG emissions has continued and in some respects accelerated. Emissions of CO2 from fossil fuels (the largest single contributor to anthropogenic radiative forcing) increased at over 3% per year from 2000 to 2010 Citation[1,2], well above long-term historic growth rates. The 2008–2009 global financial crisis scarcely dented this high growth, with a small downward blip of -1.3% in 2009, being followed by a much larger increase of well over 5% in 2010. More significantly, the long-term improvement (decline in the global carbon intensity of energy use) has reversed since the early 2000s Citation[2,3], associated with rapidly increasing coal consumption in emerging economies. These fast growing economies, together embracing approximately 80% of the global population, now emit more fossil-fuel CO2 than the developed economies, although with far lower per capita emissions Citation[1,2].
Emissions from the other major CO2 source, net deforestation, have been declining and now represent less than 10% of total CO2 emissions Citation[2]. This is foremost a rare good news story, but it also holds a warning. While reductions in deforestation constitute an important set of mitigation strategies, particularly for countries in equatorial America, Asia and Africa, the overall mitigation potential of these strategies is limited. Complete cessation of deforestation would reduce global CO2 emissions by less than 10%.
Trends in other GHGs and radiative-forcing agents are also significant. One important example is the trend in net nongaseous radiative forcing through the combined direct and indirect effects of aerosols, ozone and other processes. At present, these processes together have a net cooling effect, but many scenarios, including all the representative concentration pathways developed for the IPCC Fifth Assessment Citation[4], project that the cooling contribution will decrease strongly through the 21st century, increasing the mitigation challenge by 30–50% of present net radiative forcing.
What are the implications of these trends for meeting climate targets? Do they imply that it is too late and that exceeding 2°C of warming is now inevitable? Among the factors bearing on this important question, three stand out: biophysical capacity, the transformability of human systems and perceptions of risk.
Biophysical capacity
The Earth has a finite capacity to absorb the impacts of human activities. Important processes influencing this capacity include the uptake of atmospheric CO2 by land and ocean carbon sinks, chemical decay of reactive GHGs such as methane, the partition of the excess heat energy from radiative forcing among ocean and other heat stores and amplification of global temperature perturbations by reinforcing feedbacks through water vapor, ice extent and other mechanisms.
Sorting out the aggregate effect of all these processes is the domain of carbon-cycle and climate models, which, from the viewpoint of climate targets, are giant transfer functions mapping emissions’ trajectories (inputs) to warming and other climate consequences (outputs). Recent work Citation[5–7] has simplified the outcome of this mapping to a near-proportional relationship between cumulative CO2 emissions and warming, both measured from the start of the industrial era. This is an imperfect approximation in several ways Citation[7], as it breaks down past the time of peak temperature and extension to non-CO2 forcing is problematic, but it provides helpful guidance nonetheless. Its implication is that meeting a climate target such as 2°C depends much more on staying below a cumulative quota of CO2 emissions than on the exact path to the quota, so that more emissions in the next few years must be compensated by lower emissions in later decades, and vice versa.
A quota of the order of 1000 PgC can be emitted before the 2°C limit is exceeded with median (50%) probability Citation[5]. This is an approximate estimate with significant uncertainty (discussed below) but it provides a useful marker to characterize the challenge. More than half of a 1000 PgC quota has been emitted already through the industrial era by fossil fuel combustion and deforestation. To stay within a 2°C climate target, CO2 emissions must nearly cease after the quota is exhausted, whatever the pathway towards it. Accounting for the fact that the present growth in emissions has to be turned around, the eventual decline in emissions to meet a 2°C target has to be at a long-term ‘mitigation rate’ of more than 5% per year, if mitigation starts immediately. The required mitigation rate rises rapidly if with delay in starting mitigation Citation[7].
Transformability of human systems
Is such a decarbonization rate achievable? A global mitigation rate of 5% y-1 is comparable with the highest decarbonization rates achieved anywhere in the modern era, either by design (as in France and Sweden in the early 1980s) or unintentionally (as in the former Soviet Union in the early 1990s). However, these examples involved regional decarbonization as a side effect of some other event or process, not an intentional policy goal. They indicate that the challenge is difficult, but not that it is impossible.
The technical dimensions of the challenge involve the large-scale uptake of a range of decarbonization strategies including renewable energy, conservation and efficiency and CO2 sequestration. No option will work alone and all need to be considered, not only as modular technical problems but also as components of a fully coupled human–Earth system, because of the close coupling between carbon, energy, water, nutrients, food and human populations; examples being the interactions between bioenergy and food systems and the impacts of increasing urbanization.
Just as important as the technical issues are the human (social, cultural and psychological) dimensions of transformation. Climate futures will be shaped not only by biophysical challenges and technical opportunities, but also by human narratives, mental maps and aspirations. Our best inner narratives shape our best actions, but other narratives lead to maladaptive responses to difficult situations; for example, escapism, deferral, or various forms of wishful thinking ranging from climate-change denialism to technological over-optimism. The transformation of human systems that is needed to meet the climate challenge depends not only on technologies but also on evolution of shared narratives that can empower the necessary collective and individual actions.
Perceptions of risk
Many see the 2°C climate target as inadequate, for instance because of paleoclimatic evidence for destabilizing climate feedbacks in response to small forcings Citation[101]. This evidence calls for either or both of two responses: a tougher target such as 1°C, or a higher probability (given present uncertainties) of meeting the target with proposed actions.
The approximate CO2 quota of 1000 PgC is associated with a median or 50% probability of meeting a 2-degree target, where the probability range reflects present uncertainties in climate science. Increasing the required probability of success (P) has a major effect on targets: raising P from 50 to 80% is equivalent to lowering the temperature target (at P = 50%) by approximately 0.7°C. In terms of CO2 emissions quotas, increasing P from 50 to 80% reduces the cumulative quota to meet a 2°C target from 1000 to approximately 700 PgC (of which more than 500 PgC has been emitted already) and increases the required mitigation rate from 5% per year to an unachievable level of tens of percent per year.
Conclusion
The implication is that the combination of a 2°C warming target with high probability of success is now unreachable by conventional mitigation measures. This implies a set of difficult choices between mitigation and adaptation. Geoengineering strategies involving solar radiation management Citation[8] are not viable options: they require climate understanding and global governance at levels well beyond present human capabilities, and on present knowledge would involve heavy collateral damage to climates at regional scales.
Important as our climate choices are, the challenge extends further. Future human wellbeing depends on meeting multiple needs simultaneously, in climate, energy, water, food, ecosystem health, human health, economic functionality and spiritual fulfillment. This calls for a new relationship between humans and our finite, fragile home planet.
Financial & competing interests disclosure
The author acknowledges the support of the Australian Climate Change Science Program. This work is a contribution to the work of the Global Carbon Project (www.globalcarbonproject.org) of the Earth System Science Partnership. The author has no other relevant affiliations or financial involvement with any organization or entity with a financial interest in or financial conflict with the subject matter or materials discussed in the manuscript apart from those disclosed.
No writing assistance was utilized in the production of this manuscript.
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