A study led by Jesus Miranda published in Nature Sustainability, a top scientific journal, shows that New Zealand is in the top 10 countries, tying third, with the highest relative increase in Cooling Demand Days as the world moves from 1.5°C to 2 °C above pre-industrial levels.

What does this mean? 

The study shows how different countries and regions will be impacted by increasing cooling demands as temperature rises from 1.5°C to 2 °C above pre-industrial levels.

The 2015 Paris Agreement recognized that 1.5°C above pre-industrial levels is a critical threshold beyond which the world enters ‘dangerous climate change’.

Dangerous climate change is experienced through more intense and frequent extreme events, like storm-induced flooding. It is also manifest in hotter days that cause heat stress.

This study measures the absolute and relative increase in cooling demands due to hotter weather, using a measure called ‘Cooling Demand Days’ (CDDs).

Cooling Demand Days measure how warm a given location is and compares mean outdoor temperatures, recorded daily, with a standard temperature (usually 18°C). For example, a mean outdoor temperature of 30°C has 12 CDDs.

Should we bother?

The study shows that even a small increase in average global temperature affects heat exposure and cooling demand.

We can expect an unprecedented surge in cooling demand, and consequently energy demand. The impacts will be experienced in the Global North and South.

New Zealand is not prepared for this impact. Climate and energy policies need to build resilience to a hotter local climate and the inevitability of heat stress.

Importantly, this study adds to the robust body of knowledge already synthesized by the IPCC that clearly demonstrates that climate change is real. It is happening now.

Advancing just and climate resilient development necessitates immediate and far-reaching mitigation of greenhouse gas emissions coupled with transformational adaptation.

Cross-party agreement is necessary to enact robust legislation, with enabling policies and resourcing, to reduce climate-compounded impacts and risk.

Increasing heat stress and cooling demand adds to the litany of climate-compounded challenges in Aotearoa New Zealand. This is not what anyone wants to hear as we navigate the aftermath of multiple flood disasters.

The results of the study by Miranda and colleagues (2023) highlight the accelerating climate change impacts and implications for Aotearoa, associated with the increasingly likely 2°C of global warming. As a temperate nation, climate-related risk assessments have so far largely focused on shifting variability and mean future conditions, with recent attention to characterising the likelihood of extreme events, and cascading hazards.

As with other mid-latitude nations, many of our systems of production and distribution, and urban and rural populations are traditionally unprepared for high temperatures and large-scale adaptation to heat resilience presents a novel, complex challenge. Their findings show a dramatic, nearly 24% relative increase in the number of days in Aotearoa that may require cooling. This will affect our largest urban populations, not only through higher temperatures, but place additional demand on electricity supply, potentially straining already fragile infrastructure.

Furthermore, the direct and indirect consequences for human health, and higher costs associated with necessary cooling will not be experienced equally. Those in crowded accommodation, the urban poor, and other vulnerable members of our communities will face the greatest risk. As the recently released work from MBIE on ‘just transitions’ shows, it will be vital to consider social justice, equity and access, in adaptation and resilience strategies, including energy.

Second, these findings pose additional adaptation challenges for our primary industries, which rely on cool storage. In Aotearoa New Zealand, food production or by-products account for nearly half our exports – some 60% of which are exported in a refrigerated state. This too presents an acute risk for cold chains, which are likely to face higher energy demands and costs. Kiwifruit for example, is a $3B industry, with some 80% of fruit grown in the Bay of Plenty. With warming over the last several decades, kiwifruit are already being picked earlier, and higher temperatures will only further exacerbate this. On the one hand, the earlier ripening extends New Zealand’s market window, however much of the cool storage infrastructure dates to the 1980s boom in kiwifruit, and is designed for harvesting in May. If cool stores are activated in late-March they have more latent heat to remove from the infrastructure (concrete pads). Distribution and storage facilities may face then, the compounding effects of cooling degree days: not only the energy costs for lowering the cool store temperature, but also the energy cost for lowering the temperature of warm fruit, which must be cool and held to allow for curing (sufficient time for ethylene to dissipate from the picking wound), before going to the pack-house.

Finally, continued advances in modelling complex systems, and our ability to utilise ‘big data’, and machine learning, are even now revealing new and unprecedented challenges, with 2°C of warming. Planning and implementation of a just energy transition must begin now, and despite the uncertainty, adaptation actions which enhance resilience, should be identified and implemented. As the authors rightly conclude, “it is important to recognize that the dramatic, and often inequitable, rise in cooling demand can no longer be ignored but rather be addressed through socio-technical levers of change, which support holistic sustainable solutions.”

I am surprised to see this paper published by Nature. The authors have placed greater the relevance in terms of cooling in mid latitude climates.

Using a New Zealand example, in any one summer in Queenstown the number of days with mean temperatures above 25 deg C may be about, for example 10, with 15 cooling degree days (CDD) required. With an increase in warming from 1.5 degC to 2 degC there might be 25 CDD required: a 60% increase! However, Auckland where in summer the number of CDD, for example is much more at 40. So an increase of 10 CDD is only 25%. The take home message is that where small amounts of cooling is required, the relative increase is large. This does not mean that the amount of cooling is larger. It would be more in subtropical and tropical latitude climates, not mid-latitude climates.

We know that New Zealand is going to experience rapidly worsening heat extremes with any additional warming of the climate. The number of people over 85 years old is also expected to quadruple within the next 50 years. The big question is whether we are doing anything specific to reduce these heat-related health risks. Right now, the building code requires a minimum indoor temperature of 16°C for all care homes and early childhood centres, but there is no upper limit on allowable indoor temperatures. This is dangerous, it’s out of step with many other OECD countries, and urgently needs fixing.

The significant requirement for additional cooling will increase energy demand and costs, particularly in our urban areas which tend to be hotter anyway. The study also highlights the importance of ensuring new buildings are designed to be comfortable at these temperatures, and policies that incentivise green infrastructure and recognise the vital role that nature can play in cooling the built environment.

One thing that the study highlights is the need to understand more about how the future climate will require different patterns of energy demand and how this may play out in different geographical areas. So yes, it will increase the need for cooling, but there will also be an impact on current energy demands for warming too.