Good design can go a long way

In ancient Persia, windcatchers – or bâdgirs – were a common architectural element. These towers would catch the breeze and channel it down through the house to cool the interior. Vernacular architecture around the world has provided thermal comfort through natural heating, cooling and ventilation solutions for centuries.

In many places, we are now constructing buildings in ways that make mechanical devices indispensable, adding to the growing climate impact of heating and cooling (see box 2.1).^[27] Mechanical heat recovery systems have replaced natural ventilation in larger buildings and ACs have largely enabled us to disregard local climatic factors during building design. Meanwhile, the growing size of buildings has led to the replacement of local building materials in favor of concrete. Concrete can give rise to overheated indoor spaces in tropical regions during hot seasons, necessitating further use of ACs.^[28]

Vernacular architecture elements can still play a major role in heating and cooling decarbonization today. This approach involves reestablishing a balance with the natural elements and reintroducing tried and tested techniques in modern ways, through passive design. To encourage this process, an increasing number of countries are now establishing mandatory or voluntary building energy codes and energy efficiency standards. Active and passive strategies, particularly for reducing heating demand, are popular in North America and Europe.
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Passive heating and cooling

In cold or moderate climates, optimizing solar energy absorption through building and window orientation, insulation and improvements to the building shell are common strategies for reducing heating demand. Heat from the sun can also be absorbed by and transferred to building elements.^[29] To stay cool in summer, the simple use of shades, window screens and appropriately sized openings can often be sufficient.

In hot climates, insulation and the careful selection of sustainable construction materials with the right thermal properties could reduce the energy required for cooling by 10 to 40 percent.^[30] Depending on the region, passive cooling can also involve ventilation shafts, green roofs or reflective roof coatings and double-glazed windows as well as careful consideration of building layout.^{[31] [32]}

Energy demand for heating and cooling can also be managed at consumer level. Technology can influence behavior and nudge us toward better use of heating and cooling devices. Examples include metering systems that increase awareness and offer financial incentives for reduced energy usage.

Box 2.1 GHG emissions from heating and cooling

Heating is the largest end-user of energy, accounting for 40 percent of energy-related CO₂ emissions. In buildings, heating accounts for 80 percent of the direct CO₂ emissions. Fossil fuels still dominate, with renewables making up just 11 percent in 2022.^[33] Meanwhile, global heat consumption is expected to increase by 6 percent between 2022 and 2027.^[34] While cooling is largely electrified, the heating sector is lagging behind in electrification.

Whether for the homes, transport or industry, demand for cooling is also growing. Devices such as ACs are deeply embedded in society. In many countries, they are a symbol of rising income levels and comfort, with global warming contributing to their demand. By 2050, around two-thirds of households worldwide could have an AC (figure 2.2).^[35] In 2020, GHG emissions associated with space cooling and refrigeration represented over 10 percent of global emissions.^[36]

Energy usage accounts for 70 percent of AC emissions. The rest is due to refrigerant leakage.^[37] While refrigerants such as hydrofluorocarbons (HFCs) are not major greenhouse gases, they are the fastest growing globally.^[38] Refrigerants with low GWP are available on the market. However, this is a sector where policy intervention is crucial. Without policy intervention, emissions from AC and refrigeration are projected to rise 90 percent above 2017 levels by 2050.^[39]

 
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Heating and cooling retrofits

Natural heating, cooling and ventilation is particularly important in emerging economies with rapidly growing cities as new construction offers opportunities to consider energy efficiency from the outset. Well-designed cities could save 25 percent of heating and cooling energy usage.^[40] Meanwhile, cooling and heating retrofits are an important option in industrialized countries and cities with aging building stock. However, the rate of green retrofitting of existing buildings is low. For instance, although 75 percent of European buildings are energy inefficient, only between 0.4 and 1.2 percent of the whole stock is renovated annually.^[41]

In addition to retrofitting and enhancing energy efficiency of buildings, heating and cooling systems themselves can benefit from retrofits. Replacing conventional heating systems with heat pumps can be challenging, generally necessitating renovation for their installation. Often, significant infrastructure is already in place – central heating, piped gas networks and boilers for apartment and office buildings. Combining hybrid heat pumps with existing gas-fired boilers can minimize efficiency drops. Another transitional alternative includes retrofitting existing heating systems by replacing fossil fuel boilers with natural gas or biomass boilers fueled by agricultural and forestry residues.

However, depending on the feedstock source, biomass boilers could exacerbate competition for agricultural land needed for food production. Electrification of heating, such as through heat pumps, is a far better option from a climate perspective. Furthermore, electrification allows for integration with renewable energy sources.
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Inefficient technologies are still dominant

Scaling the use of efficient heating and cooling technologies is a key measure for reducing the climate impact of these functions. While passive design and retrofits may be more sustainable ways of managing demand, progress in these directions is not fast enough in some of the world’s most rapidly expanding cities.^[42][43][44]

Minimum energy performance standards, energy efficiency labelling and incentive programs are important measures for stimulating uptake of energy-efficient technologies.^[45] But generally, top-down regulation is sparse and, despite better alternatives being available on the market, the most commonly sold units are often several times less efficient.^[46]

While passive design and retrofits may be more sustainable ways of managing demand, progress in these directions is not fast enough in some of the world’s most rapidly expanding cities.

It is difficult to estimate the GHG mitigation potential of more energy-efficient technologies. But studies have shown that the world can avoid up to eight years of global emissions (at 2018 levels) over the next four decades through efficiency improvements and transitioning to refrigerants with a low GWP.^[47] For instance, more efficient ACs could reduce cooling energy demand by 45 percent.^[48]
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The growing impact of heat pumps

Over the past five years, global sales of heat pumps have grown at an average rate of 10 percent per year. ^[49] Europe is seeing the fastest growth (figure 2.3) and nearly half of the installed capacity today is in North America with the other half in China. In colder regions, like Scandinavia, heat pumps already meet between 40 and 60 percent of heating needs.^[50]

ACs and heat pumps are technically similar. However, heat pumps can offer both heating and cooling. They can therefore replace both fossil fuel boilers and inefficient ACs. The climate mitigation potential of heat pumps relies largely on the technology that it is replacing, the share of renewable energy in the electricity grid and the type of refrigerant used. However, the International Energy Agency (IEA) estimates that replacing fossil fuel boilers with heat pumps could reduce GHG emissions by 500 million metric tonnes by 2030.^[51]

Heat pumps and ACs transfer heat from one location to another through compression and decompression of a heat-absorbing refrigerant. Such systems can require ductwork, with pipes and tunnels running through the building. They can also be ductless, in which case they are suited for retrofitting buildings or adding heating, ventilation and air conditioning (HVAC) such as geothermal HVAC to specific rooms. Heat pumps draw heat from air, water or geothermal sources, often via separate indoor and outdoor components.^[52] While they are more efficient than ACs, they do have drawbacks such as noise, outdoor space use and exhaust heat emission which exacerbates the urban heat island effect.^[53]

The need to reduce heating and cooling energy consumption has fostered innovative new technologies. Electrocaloric heat pumps could offer refrigerant-free cooling in the future, but are still at R&D level. Other solutions, such as solar-driven heating and cooling units, could become a more common alternative sooner, especially considering that cooling units are needed most in solar-intense regions and seasons. Examples include solar concentrator collectors that are integrated with absorption chillers to provide the chilled water used in ACs, or ACs that are sold packaged with accompanying solar panels and converters. However, solar-driven systems are space consuming and typically not the most efficient or cost-effective solutions. Scaling renewable energy grid integration should be a priority.
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Alternative refrigerants

A major problem of ACs and heat pumps is the negative climate impact from refrigerants. Although regular maintenance can prevent many leaks, gases may escape when the units are repaired or decommissioned. Naturally, leakage is less of a problem when using refrigerants with a lower GWP.

The production of harmful chlorofluorocarbons (CFCs) as refrigerants in ACs and heat pumps has been phased out under the globally agreed Montreal Protocol on Substances that Deplete the Ozone Layer. While their most common replacement – HFCs – have no impact on the ozone layer, they are still potent greenhouse gases. Therefore, the Kigali Amendment to the Montreal Protocol aims to also phase out HFCs.^[54]

Replacing refrigerants with a high GWP with better alternatives could avoid 0.1 degrees Celsius of global warming by 2050.^[55] Several promising alternatives have emerged, including natural refrigerants such as ammonia, propane (R290) and CO₂. However, some refrigerants face challenges related to flammability and cost, and require technological improvements. 

Refrigerant leakage represents less than a third of the GHG emissions from cooling technologies. The rest is due to energy usage. Therefore, the use of low-GWP refrigerants alone will not be sufficient to tackle the overall climate impact from the sector.^[56] However, switching to better alternatives is a no-regret solution with low marginal abatement costs, and alternatives are available.
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From individual to district heating and cooling

As cities grow large and dense, centralized energy distribution systems such as district heating and cooling supply becomes an increasingly relevant alternative. Compared to individually installed heat pumps, boilers and ACs, district systems can reduce energy usage and costs while enabling renewable energy integration.^[57] Cities like Copenhagen, Denmark and Geneva, Switzerland have combined their district systems with heating from external heat pumps and natural cooling from surrounding seawater and lakes. Next-generation district heating and cooling systems can provide both heating and cooling simultaneously by recycling waste heat from chillers.

As cities grow large and dense, centralized energy distribution systems such as district heating and cooling supply becomes an increasingly relevant alternative

District cooling consumes between 20 and 30 percent less power than conventional alternatives. Efficiency is, however, highly dependent on the availability of natural heat sinks and vapor absorption systems as well as larger chiller systems.^[58][59] District heating systems are more common than district cooling. In 2021, more than 10 percent of buildings’ heat demand was met by district heating networks globally.^[60] However the implementation of both district heating and cooling is still relatively limited and installation is uneven across the world.^[61]
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Equal access to cooling is a growing concern

Between 1.8 and 4.1 billion people in the Global South are believed to lack access to residential cooling.^[62] In addition to structural measures to reduce people’s vulnerability to extreme heat, achieving equity in terms of cooling necessitates the provision of public spaces to cool down. Air-conditioned cooling centers, such as libraries and malls, or even dedicated chilled facilities during extreme heatwaves, are becoming important means for adapting to climate change in cities.

But local and public cooling can also be provided through urban greening.^[63] Trees and water bodies that offer temperature relief and shade are great nature-based alternatives with a multitude of co-benefits. As the urban heat island effect takes its toll on increasingly warmer cities, equal access opportunities to cool down are imperative for maintaining decent living standards and saving lives.
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Leapfrogging done right

Emerging technologies present an opportunity for countries to make significant progress toward low-carbon heating and cooling. Countries experiencing a surge in sales of ACs can benefit greatly from adopting low-GWP refrigerants and high-efficiency heaters and coolers. However, the technology choices are numerous and require ambitious investment. The right solutions must urgently be made affordable and accessible to avoid a wrong leapfrog approach and cause a lock-in effect for suboptimal refrigerants and technologies.^[64]

Similarly, the energy to fuel these solutions must be carefully chosen. For instance, many residential and district heating systems are increasing the share of biomass, often using wood pellets and wood chips. The risk of exacerbating competition for land needed to produce biomass feedstock must be considered. As appliances and systems for heating and cooling last longer than a decade, converting fossil fuel boilers to systems that burn biomass could result in a potentially damaging fuel lock-in for European countries.^[65]

Wood is still the largest biomass energy source today. However, some scientists and environmental groups have raised concerns that simply cutting down trees and burning them can release more CO₂ emissions and be even more harmful than burning fossil fuels.^[66] Growing biomass on already cleared land with measures in place to restore soil carbon can mitigate this challenge to some extent.
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