Climate Sense

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bserving

, P

redicting

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longer lead-time predictive skill. Decadal climate prediction is deter-

mined by both initial and boundary conditions. On these timescales,

deeper oceanic information and changes to radiative forcing from

greenhouse gases and aerosols play determinant roles. When consider-

ing interdecadal to centennial climate projections, not only do future

concentrations of greenhouse gases need to be taken into account, but

also changes in land cover/dynamic vegetation and carbon sequestra-

tion governed by both marine and terrestrial ecosystems.

One of the WCRP’s major challenges is to determine the limits

to predictability on the decadal timescale. Within the concept of a

unified suite of forecasts, decadal prediction bridges the gap between

predicting seasonal-to-interannual climate variability and change,

and the externally forced climate change projections over very long

periods. A focus on decadal prediction may help expedite the devel-

opment of data-assimilation schemes in Earth system models and the

use of Earth systemmodels for shorter-range prediction. For example,

seasonal predictions can be used to calibrate probabilistic climate

change projections in a seamless prediction system. Hence, there is

common ground on which to base a convergence between weather

forecast and seasonal to interannual climate prediction.

Over the past 20 years, the link between WCRP observational and

modelling efforts has been atmospheric re-analyses – these have

greatly improved the ability to analyse past climate variability. The

current climate records are usually made up of analyses of observa-

tions taken for many other purposes such as weather forecasting in

the atmosphere, or core oceanographic research. There is now recog-

nition that global climate can be understood primarily by ensuring the

collection and maintenance of quality observations of the atmosphere,

ocean and land surface (including the cryosphere). Many climate

datasets are inhomogeneous: the record length is either too short to

provide decadal scale information, or the record is inconsistent owing

to changes in sensors or operation, and absence of adequate attention

to archive and preservation of such records. As such, major efforts

were required to homogenize the observed data and make it useful

for climate purposes.

Reanalysis of atmospheric observations using a constant state-

of-the-art assimilation model has helped enormously in making

the historical record more homogeneous and useful. Indeed, in the

20 years since reanalysis was first proposed, there have been great

advances in the ability to generate high quality, temporally homo-

geneous estimates of past climate. With the ongoing development of

analysis and reanalysis in the ocean, land and sea ice domains, there

is great potential for further progress and improved knowledge of

the past climate record. There has also been some development of

coupled atmosphere-ocean data assimilation, which lays the foun-

dation for future coupled reanalysis studies, and may lead to more

consistent representations of the energy and water cycles. This latter

area is a major focus for the GEWEX project.

Another challenge confronting climate researchers is the provision

of regional-level climate information that investors, business leaders,

natural resources managers and policy makers need to help prepare

for the adverse impacts of potential climate change on industries,

communities, ecosystems and entire nations. While global mean

measurements of temperature, precipitation and sea-level rise are

convenient for tracking global climate change, many sectors of society

require actionable information on considerably finer spatial scales.

The increased confidence in attribution of global-scale climate change

to human-induced greenhouse gas emissions, and the expectation

that such changes will increase in the future, has led to an increased

demand in predictions of regional climate change to

guide adaptation.

Although there is some confidence in the large-scale

change patterns of certain predicted parameters, the skill

in regional prediction is much more limited and diffi-

cult to assess. This is because the WCRP does not have

data for a selection of different climates against which

to test models. Much research is being done to improve

model predictions, but progress is likely to be slow. In

the meantime, the WCRP recognizes that governments

and businesses are faced with making decisions now and,

as such, require the best available climate advice today.

Despite their limitations, climate models are the most

promising means of providing information on climate

change. As such, the WCRP encourages unrestricted

access to climate predictions to help in decision making

– provided the limitations of such predictions are made

clear. The WCRP has begun to develop a framework

to evaluate regional climate downscaling (RCD) tech-

niques – enabling global climate projections to be used

for regional applications.

Such a framework would be

conceptually similar to the successful coupled model

comparisons undertaken by the Working Group on

Coupled Modelling (WGCM) and would have the goal of

quantifying the performance of regional climate model-

ling techniques, as well as assessing their relative merits.

An international coordinated effort is envisioned to

develop improved downscaling techniques and to provide

feedback to the global climate modelling community. A

specific objective will be to produce improved multi-

model RCD-based high-resolution climate information

over regions worldwide for input to impact/adaptation

work and to the IPCC Fifth Assessment Report (AR5).

This would promote greater interactions between climate

modellers, those producing downscaled information and

end-users to better support impact/adaptation activities,

as well as to better communicate the scientific uncer-

tainty inherent in climate projections and information.

An important theme in this activity will be to promote

the greater involvement of scientists from developing

countries.

The WCRP will continue to provide scientific leader-

ship for major international climate assessment activities.

Currently, under the leadership of the WCRP’s WGCM,

the fifth phase of the Coupled Model Intercomparison

Project (CIMP5) is under development in support of

IPCC AR5. The grand challenge for climate models

participating in this activity is to resolve regional climate

changes to which human societies will be forced to adapt,

as well as to quantify the magnitude of the feedbacks

in the climate system. The scientific community has

formulated such coordinated experiments to address key

science questions in support of decision makers. Since

these experiments will constitute the major activity of

the international climate change modelling community

over the next few years, the results will be eligible for

assessment by AR5.

For longer timescale projections (to 2100 and beyond),

intermediate resolution (~200 kilometres) coupled