GeoLawWatch Open Legislation 11
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MTBillSB473Failed

Prohibit weather modification.

  1. Introduced
  2. Passed one chamber
  3. Passed both chambers
  4. Passed

Montana Senate Bill 473 passed the Senate in March 2025. Still, it died in the House, prohibiting large-scale geoengineering practices such as stratospheric aerosol injection and solar radiation management, while explicitly protecting cloud seeding for agriculture and water management.

Our summary

The bill prohibits geoengineering activities within Montana but makes an explicit exception for traditional cloud seeding practices used for water resource management, which remain permitted.

To clarify the scope of these restrictions, the legislation sets forth the following provisions: Specifically banned practices include stratospheric aerosol injection. Also forbidden are solar radiation management techniques like space-based reflectors and cloud albedo enhancement. Marine cloud brightening, which involves spraying sea-salt particles into marine clouds to increase reflectivity and reduce the amount of solar radiation absorbed by Earth, is also prohibited.

In addition, solar radiation management is likewise prohibited.

Marine cloud brightening is explicitly banned.

The Legislature states its rationale for these prohibitions. It justifies these actions by determining that geoengineering poses risks to the environment and public health. The risks stem from its experimental nature and potential for unintended consequences.

Explicit exception: Cloud seeding is not prohibited by the bill. Specifically, cloud seeding—defined as increasing precipitation within existing cloud systems to support water resource management, such as for agriculture and snowmaking—remains legal and continues to be regulated under Title 85, Chapter 3, of the Montana Code Annotated.

The prohibition is incorporated in Title 75, Chapter 2, of Montana's environmental laws and is subject to its enforcement provisions.

Although the bill does not specify penalties or enforcement mechanisms, it incorporates the provisions of Title 75, Chapter 2. These provisions govern environmental quality and provide a framework for enforcement. By referencing existing legal structures, the bill indirectly addresses enforcement.

Effective date: The act becomes effective immediately upon passage and approval.

Legislative history

  1. (S) Died in Process
  2. (H) 2nd Reading Not Concurred
  3. (H) Scheduled for 2nd Reading
  4. (H) Committee Report--Bill Concurred as Amended (H) Natural Resources
  5. (H) Committee Executive Action--Bill Concurred as Amended (H) Natural Resources
  6. (H) Hearing (H) Natural Resources
  7. (H) First Reading
  8. (H) Referred to Committee (H) Natural Resources
  9. (S) Transmitted to House
  10. (S) 3rd Reading Passed
  11. (S) Scheduled for 3rd Reading
  12. (S) 2nd Reading Passed
  13. (S) Scheduled for 2nd Reading
  14. (S) Reconsidered Previous Action; Placed on 2nd Reading
  15. (S) 2nd Reading Pass Motion Failed
  16. (S) 2nd Reading Indefinitely Postponed
  17. (S) 2nd Reading Motion to Amend Carried
  18. (S) Committee Report--Bill Passed (S) Natural Resources
  19. (S) Committee Executive Action--Bill Passed (S) Natural Resources
  20. (S) Hearing (S) Natural Resources
  21. (S) Referred to Committee (S) Natural Resources
  22. (S) First Reading
  23. (S) Introduced
  24. (LC) Draft Delivered to Requester
  25. (LC) Draft Ready for Delivery
  26. (LC) Draft in Assembly
  27. (LC) Draft in Final Drafter Review
  28. (LC) Draft in Input/Proofing
  29. (LC) Draft in Edit
  30. (LC) Draft in Legal Review
  31. (LC) Drafter Assigned

Additional documents

Bill text 
A BILL FOR AN ACT ENTITLED AN ACT GENERALLY REVISING LAWS RELATED TO WEATHER MODIFICATION PROHIBITING WEATHER MODIFICATION GEOENGINEERING PROVIDING DEFINITIONS AND PROVIDING AN EXCEPTION AND PROVIDING AN IMMEDIATE EFFECTIVE DATE
BE IT ENACTED BY THE LEGISLATURE OF THE STATE OF MONTANA
NEW SECTION Section 1 Prohibiting weather modification BY GEOENGINEERING 1 THE LEGISLATURE FINDS THAT GEOENGINEERING POSES RISKS TO THE ENVIRONMENT AND PUBLIC HEALTH DUE TO ITS EXPERIMENTAL NATURE AND POTENTIAL FOR UNINTENDED CONSEQUENCES
2 Except as FOR WEATHER MODIFICATION BY CLOUD SEEDING AS prescribed in TITLE 85 CHAPTER 3 a person may not intentionally inject release or disperse chemicals chemical compounds substances or apparatuses within the borders of Montana expressly for GEOENGINEERING
3 As used in this section
A CLOUD SEEDING MEANS A PROVEN METHOD OF INCREASING PRECIPITATION WITHIN EXISTING CLOUD SYSTEMS TO SUPPORT WATER RESOURCE MANAGEMENT EFFORTS INCLUDING FOR AGRICULTURE AND SNOW MAKING
B GEOENGINEERING MEANS LARGE-SCALE INTENTIONAL INTERVENTIONS IN THE EARTHS CLIMATE SYSTEM DESIGNED TO COUNTERACT OR MITIGATE GLOBAL CLIMATE CHANGE INCLUDING STRATOSPHERIC AEROSOL INJECTION SOLAR RADIATION MANAGEMENT AND MARINE CLOUD BRIGHTENING THAT INVOLVE ALTERING ATMOSPHERIC OCEANIC OR TERRESTRIAL SYSTEMS ON A PLANETARY SCALE WITH POTENTIALLY UNPREDICTABLE AND WIDESPREAD ENVIRONMENTAL CONSEQUENCES
C MARINE CLOUD BRIGHTENING MEANS SPRAYING SEA SALT PARTICLES INTO MARINE CLOUDS TO INCREASE REFLECTIVITY AND REDUCE THE SOLAR RADIATION ABSORBED BY THE EARTH
D SOLAR RADIATION MANAGEMENT MEANS MANAGING SOLAR RADIATION REACHING THE EARTHS SURFACE THROUGH STRATOSPHERIC AEROSOL INJECTION SPACE-BASED REFLECTORS OR CLOUD ALBEDO ENHANCEMENT
E STRATOSPHERIC AEROSOL INJECTION MEANS INJECTING REFLECTIVE PARTICLES INTO THE STRATOSPHERE TO REFLECT SUNLIGHT AND REDUCE GLOBAL TEMPERATURES
NEW SECTION Section 2 Codification instruction Section 1 is intended to be codified as an integral part of Title 75 chapter 2 and the provisions of Title 75 chapter 2 apply to section 1
NEW SECTION SECTION 3 EFFECTIVE DATE THIS ACT IS EFFECTIVE ON PASSAGE AND APPROVAL