It is now firmly established that human activity is causing a climate crisis. In August 2021 the Intergovernmental Panel on Climate Change (IPCC) reported a strong likelihood that the agreed target of limiting the temperature rise to 1.5°C will be exceeded, with a 2.0°C rise likely before the end of the 21^st Century. This was called a “Code red for humanity” by the UN Secretary General.

As a result of this, many of the global systems that regulate the climate are showing worrying signs of decline, potentially approaching tipping points, after which change would be irreversible.

Due to complex interdependencies and interactions of these tipping points, the global climate is an extremely difficult system to model accurately. Indeed, this year’s Nobel Prize for Physics was awarded for research that led to the development of climate models which were used to guide our response to global warming. The complexity of the climate system makes it difficult to isolate the impacts of individual causes, or to unambiguously attribute effects to climate change, but the overall trend is unmistakable.

Within this interlinked system urbanisation is a key driver of climate change.  According to the World Bank, cities contain 55% of the world’s population but are responsible for two thirds of global energy consumption and 70% of Greenhouse Gas (GHG) emissions. Urban sprawl also affects land cover by replacing green areas with pavements and buildings, causing heat islands.

To further complicate the whole story, cities are also extremely vulnerable to the effects of climate change, with many being in areas at risk from sea level rise, desertification or increased extreme weather events such as hurricanes or wildfires. This dual role as cause and victim of climate change makes cities central to any serious strategy for reducing GHG emissions.

Since its inception, Earth Observation (EO) from space has played a key role in tracking and understanding climate change. Now that more EO data is available than ever before, new ways to use this data are emerging which can address both carbon reduction strategies and climate mitigation efforts.

There are a range of ways in which satellites can address urban climate change. To start with, EO data is critical for assessing the degree of urbanization, for example, a range of products are available, such as the Global Human Settlement Layer from JRC or the Copernicus Urban Atlas, as well as more detailed assessments from commercial providers. Satellite data can also show what is being lost to cities, tracking the loss of carbon sinks (i.e. wilderness or farmland) to human activities. Land Use, Land Use Change and Forestry (LULUCF) activities are a major contributor to GHG emissions, though if managed carefully they have the potential to help remove carbon from the atmosphere.

Satellite data are also key for tracking individual emission sources, whether from cities of elsewhere. For example, Sentinel-5P can be used to monitor Methane emissions from landfill sites. Such efforts will be further boosted by the Carbon Mapper constellation currently under development.

At a smaller scale, EO data can help to monitor efforts to reduce the climate footprint of cities. For example, within e-shape there are pilot services looking at use of photovoltaic generation in urban areas and improving solar energy forecasts. Satellites have also been used for contributing to urban planning, monitoring urban green areas, and many other use cases.

A relatively novel application is to use EO data to monitor the thermal images of buildings, assessing where energy is being wasted. Although current capabilities have low resolution, there are constellations in development which are expected to be able to monitor heat loss at a building scale.

EO can also be used to support efforts to mitigate climate effects in cities. The Copernicus Climate Change service is a critical resource for forecasting the effects of changing climates on cities, such as the rise of urban heat islands, coastal flooding or increased rainfall. Such information is vital to city planners seeking to adapt to a changing climate. Within e-shape, there are pilot activities looking at assessing urban resilience to extreme weather and at the health effects on populations.

Away from EO, space resources can support cities in other ways. Technologies developed for space, such as improved batteries, can be used to improve efficiency on the ground, while positioning systems such as Galileo or EGNOS can help to improve transport efficiency, reducing emissions.

None of the applications listed above can solve climate change, which requires serious changes to patterns of transport, consumption and behaviour. Nevertheless technology in general, and EO in particular, are key tools which support cities in understanding, adapting to and reducing climate change. This role can only be expected to substantially increase in the years ahead.