CO2e is a measurement term used to measure the impact of
all greenhouse gases.
Some activities we undertake on HPC systems can potentially have
positive carbon impacts but these can be very difficult to
quantify.
Everything we do emits carbon into the atmosphere, and our goal is
to emit the least amount of carbon possible. This constitutes the first
principle of green HPC system use: carbon efficiency, emitting
the least amount of carbon possible per unit of work.
In regions where electricity production is dominated by burning
fossil fuels, electricity is a good proxy for carbon, so using HPC in an
energy efficient way is equivalent to using HPC in a way that is carbon
efficient.
In regions of where electricity production is dominated by low
carbon energy sources, electricity is not a good proxy for carbon.
Green HPC use takes responsibility for its electricity consumption
and considers how this relates to carbon emissions.
Quantifying the energy consumption of your HPC use is a step in the
right direction to start thinking about how you can operate more
efficiently. However, understanding the energy consumption of your use
of HPC is not the only story. The hardware your software is running on
uses some of the electricity for operational overhead. This is called
power usage efficiency (PUE) for HPC systems (and for computing
resources hosted in data centres more generally).
How you go about improving the energy efficiency of use of an HPC
system depends on the software you are using and the input parameters as
well as the hardware.
Often, the only practical way forward in the face of this complexity
is to perform some benchmarking to assess how energy efficiency can be
improved.
Carbon awareness means understanding that the energy you consume
does not always have the same impact in terms of carbon intensity.
Carbon intensity varies depending on the time and place it is
consumed.
The nature of fossil fuels and renewable energy sources means that
consuming energy when carbon intensity is low increases the demand for
renewable energy sources and increases the percentage of renewable
energy in the supply.
Demand shifting means moving your energy consumption to different
locations or times of days where the carbon intensity is lower.
Demand shaping means adapting your energy consumption around carbon
intensity variability in order to consume more in periods of low
intensity and less in periods of high intensity.
Embodied carbon is the amount of carbon pollution emitted during the
creation and disposal of an HPC system.
When calculating your total carbon pollution, you must consider both
that which is emitted when running the on the HPC system as well as the
embodied carbon associated with its creation and disposal.
Extending the lifetime of an HPC system has the effect of amortising
the carbon emitted so that its embodied CO2e/year is
reduced.
Increasing utilisation and performance also improve the embodied
carbon efficiency from HPC system use.
The GHG protocol is a metric for measuring an organisation’s total
carbon emissions and is used by organisations all over the world.
The GHG protocol puts carbon emissions into three scopes. Scope 3,
also known as value chain emissions, refers to the emissions from
organisations that supply others in a chain. In this way, one
organisation’s scope 1 and 2 will sum up into another organization’s
scope 3.
You can use the GHG protocol to estimate your emissions from HPC
system use but it requires access to reasonable quality information from
the HPC systems you are using.
The HPC-CI is a metric designed specifically to calculate emissions
from HPC systems and is a rate rather than a total. This can be used to
measure improvements in emissions efficiency and drive reductions in
emissions.
There are a number of methodologies commonly applied to help in the
overall fight against climate change. These fall into the general
categories of carbon elimination (also known as “abatement”), carbon
avoidance (a.k.a. “compensating”), or carbon removal (a.k.a.
“neutralising”).
Abatement is the most effective way to fight climate change although
complete carbon elimination is not possible.
Compensating includes the adoption of renewable energy sources,
sustainable living practices, recycling, planting trees etc.
Neutralisations refer to the removal and permanent storage of
atmospheric carbon to counterbalance the effect of releasing
CO2 into the atmosphere. Neutralisations tend to remove the
carbon from the atmosphere in the short and medium-term.
Net zero aims to eliminate emissions and only offset the residual
emissions that you cannot eliminate to reach the 1.5°C target set by the
Paris Climate Agreement.
Which strategies users or HPC system operators prioritise to reduce
emissions depends on if the operational or embodied emissions
dominate.
A key part of reducing emissions from HPC use is reducing our
consumption of HPC resources.
