DEFINING DANGEROUS

Can we define an “acceptable” or “dangerous” level of climate change?

Defining an “acceptable” or “dangerous” level of climate change is not a simple technical calculation, as it involves both scientific data and human values. While science can project the potential impacts of different levels of warming, the decision of which impacts are tolerable is ultimately a societal choice

→ Science aims to provide positive (or objective) judgements based on experimentation, evaluation of data and hypothesis testing -> the scientist itself is not objective, but the way they are gathering data is systematic and objective

 → The choice of action is a normative judgement. The choice depends on values, which may be cultural (a way we live), economic (cost, discount rate?)

Is what we define as acceptable or unacceptable climate a scientific question?

While scientists provide the evidence used to inform these definitions, the question itself is not strictly scientific. Instead, it is a negotiation between science and policy.

• Positive Judgements: Science aims to provide positive judgements, which are objective statements based on experimentation, the evaluation of data, and hypothesis testing. For example, a positive judgement might state that 2°C of warming will lead to the loss of 99% of coral reefs.

• Normative Judgements: Deciding whether the loss of those reefs is "acceptable" or "dangerous" is a normative judgement. These are choices based on values, such as cultural norms, ethics, or economic priorities. Because people and nations value different things—such as economic growth versus the preservation of low-lying atoll nations—there is no single scientific definition of what constitutes an "acceptable" risk

dangerous interference with the climate system

= the central focus of international climate policy, such as the United Nations Framework Convention on Climate Change (UNFCCC). It refers to human-caused changes that are significant enough to destabilize the climate resource we all share

what qualifies as this:

• Physical and Ecological Impacts: These include the melting of the Greenland or West Antarctic ice sheets, widespread loss of coral reefs, or the disruption of the South Asian Monsoon.

• Climate Feedbacks: This involves triggers that could accelerate warming, such as the release of methane from melting permafrost or the drying out of the Amazon rainforest.

• Lives and Livelihoods: This focuses on human costs, such as the depopulation of atoll countries due to sea-level rise, millions of people facing hunger or water shortages, and the potential for conflict or international instability

temperature targets as boundary objects

Because there is no fixed physical threshold where the climate suddenly becomes "dangerous," the international community uses temperature targets like 1.5°C and 2°C.

• Origin of Limits: The 2°C limit was first suggested by an economist in 1975 and later adopted by the European Union and the UN.

• Boundary Objects: These targets are considered “boundary objects,” meaning they are arbitrary geopolitical markers that allow scientists, policymakers, and experts from different fields to negotiate and coordinate action.

• Scientific Reality: From a scientific perspective, there is no true threshold beyond which a catastrophe occurs; rather, every bit of warming increases the risks and severity of impacts. The 1.5°C goal is a "guardrail" intended to avoid the most severe outcomes, particularly for the most vulnerable populations, such as the poor and disenfranchised

How were the 1.5 and 2 °C temperature limits chosen?

were not derived from a single scientific discovery but emerged through a long history of economic suggestions, scientific assessments, and international political negotiations:

• The 2 °C Limit: This target began with a 1975 suggestion by economist William Nordhaus, who used what he described as a "deeply unsatisfactory" process to identify it as a potential limit. It was later supported by European scientists as a "next best" limit (after 1 °C) and was proposed by the European Union in 1996. It was finally enshrined in international policy at the 2009 UN climate summit in Copenhagen.

• The 1.5 °C Limit: This lower target was first mentioned in the 2010 Cancun Accord as a goal to strengthen the long-term global target. It became a central part of international policy in the 2015 Paris Climate Agreement, which committed nations to holding warming "well below 2 °C" while pursuing efforts to limit it to 1.5 °C

Are the temprature limits a product of science alone?

No, these limits are not the product of science alone; they are negotiated "boundary objects".

• Positive vs. Normative Judgements: Science provides positive (objective) judgements—such as what impacts will occur at specific temperatures—but deciding which of those impacts is "acceptable" is a normative judgement based on human values, ethics, and economics.

• Arbitrary Markers: These targets are arbitrary geopolitical markers that allow scientists and policymakers to coordinate action. From a scientific perspective, there is no true physical threshold beyond which a catastrophe suddenly occurs; rather, every bit of warming increases the risk

UNFCCC and IPCC and dangerous interference with the climate system international negotiations

The framework for these negotiations is the United Nations Framework Convention on Climate Change (UNFCCC), established in 1992.

• Core Objective: The ultimate goal of the UNFCCC is the stabilization of greenhouse gas concentrations at a level that prevents “dangerous anthropogenic interference (DAI) with the climate system”.

• The Role of the IPCC: The Intergovernmental Panel on Climate Change (IPCC) provides the scientific evidence to inform what qualifies as "dangerous". While the IPCC characterizes risks using tools like the "burning embers" diagram, they are charged with being policy-relevant but not policy-prescriptive, meaning they provide the data but do not choose the targets themselves

implications of passing the warming limits

Passing these warming limits does not mean the world ends on a specific date (like 2030), but it does signify a transition into much higher risk levels.

• Escalating Risks: The IPCC warns that risks to health, food security, water supply, and economic growth increase significantly at 1.5 °C and even more severely at 2 °C.

• Non-linear Impacts: Many climate impacts are non-linear, meaning small increases in temperature can lead to large, sudden changes in impacts.

• Current Trajectory: Experts conclude that at least 1.5 °C of warming is likely inevitable given current emissions and the lag between warming and its impacts. Avoiding these limits now relies heavily on negative emissions technologies, which are largely unproven at scale