The enigma of the 1.5C carbon budget, intricately woven into the fabric of the Paris Agreement, has sparked a global debate. As nations commit to “pursue efforts to limit the temperature increase even further to 1.5C,” the question remains: What does this truly entail? In this comprehensive exploration, we delve into the nuances of the 1.5C challenge, dissecting the uncertainties, and deciphering the latest research on carbon budgets.

Unpacking the Ambiguity

The Paris Agreement, a milestone in the battle against climate change, provides a clear directive to limit global warming. However, it falls short of defining the specifics of the 1.5C target.

Is it about achieving a 1.5C warming with a 50% chance of remaining below it? Or does it entail striving for “well below” 1.5C, similar to the 2C goal, with a 66% chance of avoiding a temperature rise beyond 1.5C?

Moreover, the agreement leaves room for interpretation regarding the reference point. Does it refer to changes in the global air temperature as simulated in models, or is it a blend of air temperature and ocean surface temperature, aligning more closely with observational records?

The Battle of Percentages

The debate rages on with some reports focusing on a 50% chance of staying below 1.5C, while others advocate for a 66% chance of avoiding the dreaded threshold. The tide appears to be turning in favor of the “well below” 66% interpretation, primarily because the Integrated Assessment Models (IAMs) used in preparation for the next IPCC report have embraced this approach.

The Carbon Budget Conundrum

The heart of the matter lies in the carbon budgets. Recent studies have examined the prospects of restraining warming to 1.5C with a 50% chance. These studies also explore the “well below” 66% probability. Two novel studies stand out in this arena, shedding light on alternative methodologies.

Out with the Models, In with Observations

Prof. Damon Matthews and colleagues, along with Millar and Friedlingstein, offer a departure from traditional carbon budget calculations that heavily rely on models. Instead, they venture into uncharted territory by estimating the correlation between observed warming and cumulative CO2 emissions. Their key metric is the “transient climate response to cumulative emissions,” quantifying the amount of warming per teratonne carbon (TtC or 1000 gigatonnes of carbon).

The outcomes of these studies reveal a fascinating revelation. Their estimates of the transient climate response to cumulative emissions are generally smaller than those predicted by climate models. Consequently, this leads to a larger carbon budget.

However, it’s worth noting that results remain somewhat sensitive to the choice of temperature records. For instance, Millar and Friedlingstein report a transient climate response of 1.84C/TtC using the Cowtan and Way temperature record, 2.05C/TtC using the Berkeley Earth temperature record, and a median estimate of 2.11C/TtC using CMIP5 ESMs. Conversely, these values decrease when considering new 2017 estimates from the Global Carbon Project, which account for higher historical land-use CO2 emissions.

Estimates Paint a Different Picture

Matthews and colleagues peg the remaining 50% 1.5C carbon budget from January 2018 at 977 GtCO2 (equivalent to 24 years of current emissions), while Millar and Friedlingstein propose a figure of 835 GtCO2 (equivalent to 20 years of current emissions). These values starkly contrast with the IPCC earth system model value of 268 GtCO2 (equivalent to seven years of current emissions).

Another study led by Nicholas Leach and colleagues at Oxford University adopts a similar methodology. It combines estimates of both the level and rate of human-induced warming while keeping the contribution of non-CO2 factors constant. The results provide a 1.5C carbon budget range of 479 to 1239 GtCO2, translating to 12 to 31 years of current emissions, with a best estimate of 839 GtCO2.

Navigating Uncertainty

The landscape of carbon budget research is vast, with a plethora of studies published over the last two years. While these studies generally align with the conclusion that IPCC models underestimate the remaining carbon budget, significant variations persist. Pinning down a precise number for allowable emissions remains elusive due to the sensitivity of various factors, such as observed warming to date and future non-CO2 emissions.

IAMs vs. ESMs

IAMs and ESMs offer differing perspectives. IAMs primarily target warming in 2100, occasionally overshooting the 1.5C mark in the interim. In contrast, ESMs focus on the threshold of CO2 emissions before temperatures breach 1.5C. It’s important to recognize that comparing the two directly is complex.

At this juncture, it’s premature to conclude that IAM carbon budgets are too conservative. The intricate web of factors influencing carbon budgets keeps the debate alive and well.

The Practical Utility of a Carbon Budget

Ultimately, the concept of a remaining carbon budget may not be as straightforward as it seems, especially for stringent emission targets like 1.5C. The global temperature is already teetering close to the 1.5C mark, making precise calculations all the more critical.

Moreover, as nearly any plausible scenario necessitates a significant volume of negative emissions later in the century, the carbon budget becomes more of a guideline than a strict limit. Regardless of the specific carbon budget employed, the world is edging closer to the imperative task of achieving net-zero and eventually net-negative emissions.

In conclusion, the 1.5C carbon budget debate is a testament to the complexities of climate science. As we navigate the uncertain terrain of climate targets, it’s evident that mitigating the impacts of climate change requires immediate, concerted effort and a commitment to environmental stewardship.