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Climate change, oceans, and the importance of multiple indicators

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Posted April 1, 2015

Last fall, the climate policy world received a wake-up call when Nature magazine ran a commentary by David Victor and Charles Kennel, provocatively titled “Ditch the 2 °C warming goal.” The authors argued that instead of relying exclusively on global average temperature as our climate policy focus, researchers and policymakers should instead consider a suite of climate indicators, or planetary “vital signs,” to understand how the Earth is responding to rising CO2 levels.

Long Island Sound from the Thimble Islands in Branford, CT. Image credit: Ariana Spawn

Long Island Sound from the Thimble Islands in Branford, CT. Image credit: Ariana Spawn

The authors point to the ocean as an important source of new indicators because of its critical role in mediating the effects of a warming climate. Estimates suggest the ocean absorbs about 90% of the extra energy introduced to the climate system as a result of anthropogenic activities. Victor and Kennel believe this is why we haven’t witnessed a more substantial increase in global average temperature over the past few decades. Because of this function, the authors specifically highlight ocean heat content—a measure in joules of how much heat is stored in the ocean—as a better indicator of climate change than global average temperature.

Sea level rise is by far the most oft-discussed example of how the ocean is responding to climate change. Rising seas are a critical issue. Since so much of the world’s population and infrastructure is concentrated along the coasts, it may be one of the most powerful impacts of a warming climate on peoples’ lives. But sea level rise is not the only change happening in the world’s oceans: it’s also getting warmer and more acidic, and both of these impacts have major implications both for marine life and for our understanding of how the planet is responding to climate change.

The ocean warms by absorbing energy from sunlight and storing it as heat. Scientists measure ocean heat content, the indicator that Victor and Kennel highlighted, by using calculations of ocean temperatures from different depths around the world. And just as the ocean absorbs the sun’s energy, it also absorbs CO2—about a quarter of the world’s total emissions in total. Through chemical reactions, rising CO2 content in the ocean turns the water more acidic. This in turn creates a more hostile environment for marine organisms, including many economically important species of shellfish, which can’t build shells in high-acidity conditions.

Toward new indicators

Assessments of ocean acidity have historically had to rely on a combination of point measurements and modelling—until February of this year, when an international team of scientists announced a new technique for monitoring global ocean acidity with satellites. This new technique combines measurements of ocean temperature and salinity to predict ocean alkalinity, which is related to acidity levels in the water. This allows faster and more large-scale monitoring of ocean acidification than was previously possible. Such global-scale monitoring capabilities equip researchers and policymakers with a new tool for understanding how ocean acidity is changing in different regions, and it could help inform management decisions and priority-setting by highlighting specific ocean regions that are experiencing the highest relative degrees of acidification.

While changes in ocean heat and ocean acidification both result from the same driver—rising levels of atmospheric CO2—and are both assessed with the help of measurements of ocean temperature, the two processes have different implications for ocean life. Species possess a broad range of abilities for responding to rising temperature and acidity levels, and examining ocean change highlights how just how heterogeneous the effects of climate change are. For example, even though global average temperature and ocean heat content are rising as a result of climate change, they are not rising at identical rates (see Figure 1 again).

The proposal to shift away from an exclusive focus on global average temperature as the primary indicator of climate change has widespread policy and management implications. For starters, a focus on ocean-based indicators like ocean heat content and ocean acidification could heighten public awareness of the impacts of climate change on marine systems. In addition, tracking an indicator like ocean heat may contribute to a more accurate understanding of long-term climate trends, as the heat energy stored deep in the ocean is likely to be gradually released into the atmosphere over decades or centuries. Finally, focusing more intently on a broader suite of climate indicators or “vital signs” will ultimately result in a more comprehensive understanding of the way that climate change is impacting the global environment.

Because the EPI is a country-level index, our current ocean metrics necessarily focus on place-based coastal indicators that fall within countries’ national jurisdictions. However, the EPI team is actively thinking about how to improve our country-specific ocean metrics. Although measures of ocean heat and ocean acidity can’t be attributed to an individual country’s environmental performance, the burgeoning conversation about alternative methods for measuring climate change effects in the ocean illustrates the importance of having a diverse set of indicators on our radar.

Source: Yale University

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