In 2006 Spash was appointed as Chief Executive Officers’ Science Leader at the CSIRO, Australia's federal government agency responsible for scientific research.

After finishing a critical paper about emissions trading - which had already passed the peer review process - the agency intervened and pushed for substantial changes.

[8] Spash was one of the earliest economists to pay attention to human induced climate change, pioneering an alternative economics of the environment.

He followed some aspects of the work of his doctoral supervisor, Ralph C. d’Arge, in exploring its economic and ethical implications with respect to intergenerational equity and justice.

[18] Spash’s work has developed in devoting attention to the social economic consequences of a broad range of environmental problems (e.g. acidic deposition, tropospheric or ground level ozone, greenhouse gases, conservation, ecosystem and species protection).

He has also importantly incorporated a history of thought and philosophy of science perspective in his reimagining of the ontological and epistemological foundations of economics as a field.

[34] This research moved from work on acidic deposition to tropospheric zone to greenhouse gases and climate change, as evident in his dissertations and thesis.

The approach to intergenerational ethics that is reduced down to a discussion over discount rates is exposed as obscuring the presence of implicit value judgements while claiming objectivity.

[39] He employed contingent valuation to conduct empirical research in innovative ways that included ethical motives as well as attitudes and norms.

[40][41][42][43][44] At the same time his work revealed fundamental failures of contingent valuation research [45] and its employment for public policy by environmental economists.

[46] He moved into exploring the possibilities for alternative deliberative approaches,[47][48] their combination with monetary valuation,[49][50] and the implications of deliberation for economic value theory.

[81] His work here developed approaches to empirically investigate ethically motivated refusals to trade-off species and ecosystems for money as expressed by the occurrence of lexicographic preferences[82][83][84] .

[87][88] In the early 1990s Spash designed a survey conducted in Scotland on forestry to explore the valuation of biodiversity and the occurrence of protesting and refusals to make the trade-offs that economists had previously assumed were rational.

[95] When "Biodiversity Economics: The Dasgupta Review" appeared in 2021, advocating natural capital, Spash produced critical deconstructions of the report.

[101] His key conclusions here support the need for integration of social, ecological and economic knowledge,[102] and combining heterodox schools of thought in a structured methodological pluralism.