To avert the worst impacts of climate change, from extreme flooding to devastating droughts, the world will need to cap global warming at 1.5 degrees Celsius, according to the latest United Nations IPCC Report on the Earth’s climate system. Achieving that goal means that by around 2050, the planet’s total greenhouse gas emissions will need to decline to net-zero. To that end, more and more governments and businesses are setting net-zero emissions targets.   

At the XLIV (44th) MIT Global Change Forum on March 23-24, 2022, more than 100 attendees from industry, academia, government and NGOs gathered at the Samberg Conference Center on the MIT campus and on Zoom to explore how global net-zero emissions goals are creating challenges and opportunities for carbon budgets, decarbonizing energy and industry, nature-based solutions, climate and health, negative emission technologies, and policy design. Facilitated by the MIT Joint Program on the Science and Policy of Global Change in an informal, “off-the-record” setting for independent assessment of studies and policy proposals, presentations and discussions examined this year’s Forum theme from a variety of perspectives.

"We meet at a time when the urgent need to transition to a net-zero-greenhouse-gas-emitting world is made even more complex by the global COVID-19 pandemic, the premature acceleration of climate extremes, and now the Russian invasion of Ukraine,” said MIT Joint Program Director Ronald Prinn, a professor at MIT’s Department of Earth, Atmospheric and Planetary Sciences in his opening remarks. “New questions now arise such as how an emerging case for security in national energy supplies may help or hinder the net-zero transition. As the complexity grows, the need for deep-dive modeling of complex interacting human and natural systems that is the hallmark of the Joint Program on the Science and Policy of Global Change is becoming more and more evident."

Here, with permission from all speakers, we summarize key points from this year’s Forum presentations.

Carbon Budgets

The first session explored the concept of carbon budgets and how it can be applied in the design of strategies aimed at achieving net-zero-emissions.

One common definition of a carbon budget is “the total net amount of carbon dioxide (CO₂) that can still be emitted by human activities while limiting global warming to a specified level.” The impetus for estimating the Earth’s “remaining-carbon budget” is that concentrations and growth rates of CO₂—the main driver of long-term anthropogenic climate change—are the highest they’ve been in millions of years. The latest IPCC Report estimates that there’s a 50% probability that we can limit global warming to 1.5°C (or 2°C) starting in 2020 with a carbon budget of about 500 gigatons (Gt) (or 1,350 Gt) of CO₂. Another carbon budget definition quantifies exchanges and storage of carbon between and within global land, ocean and atmosphere systems. While about half of CO₂ emissions get sequestered in land and ocean systems, the remaining half ends up in the atmosphere where it largely warms the global climate along with other, shorter-lived greenhouse gas emissions such as methane. In recent years, the ability of the land and oceans to store CO₂ has showed signs of weakening, a trend consistent with El Nino Southern Oscillation events and evidence of climate-warming impacts from Earth-system models.

To estimate a remaining-carbon budget, the IPCC considers: historical warming to date (about 1.1°C), transient climate response to cumulative emissions of CO₂, zero-emission commitment (how much warming might still occur if emissions go to zero), projected future non-CO₂ temperature contribution, and unrepresented Earth-system feedbacks—all accompanied by uncertainty ranges. Estimated carbon budgets determine how much CO₂ can still be emitted in order to align with a specified climate target. They also provide the scientific basis for net-zero targets. While many of today’s announced net-zero targets are imprecise, they can be improved by providing clarification on scope, adequacy and fairness, and the long-term roadmap for achieving the target. By using cumulative emissions until net-zero to design mitigation pathways, limitations of the current scenario literature can be overcome—reducing the risk of exceeding maximum temperature limits and limiting the burden on future generations to remove large quantities of CO₂ from the atmosphere.  

Decarbonizing energy and industry

The second session focused on how the energy and industry sectors can effectively and efficiently reduce greenhouse gas emissions in alignment with net-zero emissions goals.

The energy sector contributes about 73 percent of global greenhouse gas emissions. To achieve net-zero emissions by 2050, the sector must decarbonize at an unprecedented pace. But to be deployed at scale, zero-carbon energy technologies must not cause significant increases in energy prices and declines in energy access. Among these are wind and solar, which now account for two percent of global primary energy use and must increase dramatically. The MIT Joint Program, most notably in its 2021 Global Change Outlook, has explored different emissions pathways and risks in the coming decades. Its most ambitious climate policy scenarios show a substantial decline in fossil fuel use, and significant increases in wind and solar, and in electrification. A wide range of future technologies will be needed to get to net-zero, from advanced nuclear power to direct air carbon capture. Critical minerals will be in greater demand for the clean energy transition, and obtaining sufficient quantities could be a challenge.

A recent net-zero emissions (NZE) scenario prepared by the International Energy Association (IEA) shows that dramatic reductions in industrial CO₂ emissions will be needed to achieve net-zero emissions from the energy sector by 2050. One key challenge is posed by heavy industries—primarily steel, cement and chemicals—particularly in emerging market and developing economies, where they are expected to produce the majority of industry-sector emissions in 2050. Heavy industries use large amounts of fossil fuels, especially to generate high-temperature heat for industrial processes. The IEA NZE scenario shows that interventions at end of the next 25-year capital investment cycle could prevent the release of about 60 gigatons of cumulative CO₂, around 40% of projected emissions from existing heavy industry assets. While direct substitution of electricity at the scale required is impractical or expensive with today’s technologies to reduce heavy-industry emissions, innovative technologies such as hydrogen and carbon capture utilization and storage could play a critical role.

Nature-based solutions

The third session examined how nature-based solutions (NBS) can contribute to global efforts to achieve net-zero emissions.

The World Conservation Union defines nature-based solutions as “actions to protect, sustainably manage, and restore natural or modified ecosystems, that address societal challenges effectively and adaptively, simultaneously providing human well-being and biodiversity benefits.” NBS opportunities include protecting natural ecosystems, restoring degraded ecosystems, and more sustainably managing ecosystems used for food, fiber and energy production. One NBS method, reforestation, could deliver substantial CO₂ sequestration, but also intensify competition for land-based food production. Agroecological farming, another NBS approach, may store 20-33% more soil carbon than conventional agriculture, but runs the risk of mal-adaptation and mal-mitigation. Finally, systems to monitor and measure carbon sequestration will be needed to determine how much to pay NBS providers for the environmental services they perform. 

NBS that is implemented within large-scale systems and in ways that also meet human needs can be at least as additional and “permanent” as reductions in fossil fuel extraction. To that end, there is an urgent need to act now on deforestation to avoid nearly irreversible loss. Beyond avoiding tropical deforestation, there is a lot of global potential for NBS carbon storage through afforestation/reforestation, and soil carbon sequestration in croplands and grasslands. NBS could contribute 29% of net reductions needed to be on a 2°C pathway in 2030, but one key challenge is to make NBS crediting programs effective and equitable. In one analysis, the global use of carbon markets with forest-based NBS could allow nearly doubling of climate ambition at the same cost, relative to current Paris Agreement pledges. Jurisdictional approaches to forest protection, in which deforestation is reduced through national or regional-scale forest protection programs, could provide high-integrity credits from avoiding tropical deforestation.

Climate and health

The fourth session centered on efforts to formulate integrated emissions-reduction policies that not only help stabilize the climate but also improve air quality and public health outcomes.

Fine particulate matter (PM_2.5) resulting from the combustion of fossil fuels contributes to more than 25 percent of all air pollution-related deaths globally. The use of solid fuels (wood, charcoal and animal dung) in residential settings is another major contributor to air pollution-related health impacts. Policies that accelerate a transition away from fossil fuels and toward clean energy sources could improve air quality and public health outcomes considerably while simultaneously advancing climate goals. The changing climate is expected to increase public health vulnerability and costs, underscoring the need to incorporate air quality and health concerns in climate action. Key questions that can advance integrated air pollution, public health and climate policies are: What are the major sources of air pollution and greenhouse gases, and how do they contribute to health impacts; what are their relative contributions to disease burdens; and what actions are needed to achieve substantial improvements in the future?

Previous research in this space separated “direct” from “indirect” benefits of climate policies, framing improved health outcomes as “co-benefits” of such policies. But a more holistic approach to policy design could advance an integrated set of objectives based on questions such as: What are the observed impacts of climate and energy policies on air quality? Who benefits and why (including assessment of environmental justice and equity)? What strategies can promote well-being for the present and future (and what new methods and models are needed to evaluate options)? This approach could yield new insights on proposed energy, climate and air pollution policies such as: health impacts may depend on local responses to policy; and maximizing overall benefits at the national level may not address disparities at subnational levels. New models and methods can facilitate multi-dimensional assessment (e.g. of multiple indicators/outcomes relevant to sustainability) of policy strategies on different scales. 

Keynote address: MIT Grand Climate Challenges

The keynote address highlighted the MIT Grand Climate Challenges initiative, which seeks to “mobilize the MIT research community to develop game-changing solutions to the most challenging unsolved problems in climate adaptation, mitigation and restoration.” Engaging all disciplines across MIT, the initiative aims to draw on the MIT innovation ecosystem and develop new partnerships with multiple communities, businesses and investors to accelerate development, field-testing, implementation and scaling of these solutions. Twenty-seven finalist projects represent four themes: building equity and fairness into climate solutions; removing, managing and storing greenhouse gases; decarbonizing complex industries and processes; and using data and science to forecast climate-related risk. In the spring of 2022, MIT will announce a small number of flagship projects from among the 27 finalists.

Negative emission technologies

The fifth session explored the potential of negative emission technologies to enable the world to meet net-zero emissions and long-term Paris Agreement climate targets.

Negative emission technologies (NETs) are those that physically remove carbon dioxide from the atmosphere and store it in a manner intended to be permanent, with the total quantity of stored CO₂ exceeding the total quantity of CO₂ emitted or leaked into the atmosphere by the NET. NETs include afforestation and reforestation, soil carbon sequestration, biochar, bioenergy with carbon capture and storage (BECCS), direct air capture, enhanced weathering and ocean alkalinization, and ocean fertilization. NETs are not an alternative to greenhouse gas mitigation methods, but a complementary toolset to help ensure that emissions and climate targets are met. How much the world will need to rely on NETs to meet those targets will depend on how late it starts to aggressively mitigate emissions at the global level. Within the portfolio of NETs, no one method is a silver bullet. To ensure climate stabilization, NETs must be deployed in such a way that the CO₂ that they extract from the atmosphere is removed permanently and is subject to effective measurement, reporting and verification (MRV) protocols.

A study of BECCS designed to quantify its potential scale and impact on the economy under 1.5°C or 2°C scenarios shows that in 2100 without BECCS, total primary energy (TPE) is 33-38 percent of what it would be in a business-as-usual (BAU) scenario; with BECCS, TPE nearly reaches BAU levels, with emissions from oil use offset by BECCS. When it comes to net CO₂-equivalent emissions under a 1.5°C or 2°C scenario, without BECCS the world will need significant additional emissions reductions; with BECCS it will have a lot more “headroom” to achieve the same emissions pathway. The study shows that BECCS significantly reduces the cost of meeting long-term targets, causes significant land-use change, but only increases food prices by about 1.5 percent. All technical components for large-scale BECCS now exist, but many challenges, from availability of sustainable biomass to public acceptance, could limit its deployment. Other research indicates that when designing climate-stabilizing emissions pathways, one must consider the full range of options (no NETs to multiple NETs) for risk assessment and planning.

Policy: The way forward

The sixth and final session explored the design and implications of policies aimed at achieving net-zero emissions targets, with a focus on the near-term actions needed to get there.

Prospects for greenhouse gas (GHG) mitigation in the United States improved in 2021 with the passage of the Infrastructure Investment and Jobs Act and the House of Representatives’ passage of the Build Back Better (BBB) Bill. Stalled in the Senate, BBB would earmark $555 billion for measures aimed at reducing GHG emissions 50-52-percent below 2005 levels by 2035. While the bill focuses on many sectors of the economy, it would reduce emissions the most in the transportation and electricity sectors.

Meanwhile, the European Green Deal led to the enactment of a European Union law that seeks climate neutrality by 2050 and sets the EU’s Paris Agreement target for 2030 to at least 55 percent below 1990 GHG emissions levels. The EU also introduced a “Fit for 55” package of 16 legislative proposals aligned with that target, and a Sustainable Finance Framework to re-orient capital flows toward sustainable investment. Finally, the EU is working to phase out dependence on Russian fossil fuel imports.

Recent successes in decarbonizing the energy sector provide lessons for mitigation of GHG emissions in agriculture, forestry and other land use (AFOLU). What has accelerated decarbonization in the power sector—technical advances, simulation modeling, policy support and institutional innovation—might also bring about the level of innovation and investment needed to substantially cut GHG emissions in AFOLU. Accounting for about 21 percent of global GHG emissions in 2018, AFOLU is a major source of methane emissions, and is the only sector with significant potential to deliver net-negative emissions.

In 2020, the Governor of the Commonwealth of Massachusetts committed the state to a 2050 net-zero greenhouse gas emissions goal; in 2021 he signed into law An Act Creating a Next Generation Roadmap for Massachusetts Climate Policy, which codified that goal. Analysis conducted by the Massachusetts Executive Office of Energy and Environmental Affairs found that the state could achieve the 2050 goal cost-effectively and equitably. Strategies include deployment of large-scale offshore wind, importation of additional hydropower, and decarbonization of home heating systems and private vehicles—and ensuring that the needed green technologies are adopted by and affordable for everyone.

Photo: Deployment of offshore wind at utility scale is one of many strategies to reduce greenhouse gas emissions in alignment with net-zero emissions targets. (Source: Jesse Costa/WBUR)