| 4SS | The four seasons—spring, summer, fall (autumn), and winter. | |
| ABC | Anything But Climate | |
| ACACIA | Arid Climate Adaptation and Cultural Innovation in Africa | |
| ACAN | Architects Climate Action Network | |
| ACASA | Atlantic Climate Adaptation Solutions Association | |
| ACC | Automatic Climate Control | |
| ACCC | Advisory Committee on Climate Change | |
| ACCC | American Cities Climate Challenge | |
| ACCTS | Agreement on Climate Change Trade and Sustainability | |
| ACDC | Advanced Climate Dynamics Courses | |
| ACG | Alberta Carbon Grid | |
| ACIS | Applied Climate Information System | |
| ACJP | Australian Climate Justice Program | |
| ACME | Accelerated Climate Modeling for Energy | |
| ACME | Accelerated Climate Model for Energy | |
| ACPC | Aerosols Clouds Precipitation and Climate | |
| ACRE | Agricultural Climate Resiliency Efficiencies | |
| ACRE | Alliance for a Climate Resilient Earth | |
| ACRII | Accelerating Climate Resilient Infrastructure Investment | |
| ACSIS | Atlantic Climate System Integrated Study | |
| ACT 25 | Allied Climate Transformation (COP 25) | |
| ACTT | A Climate To Thrive | |
| ADVICE | Annual to Decadal Variability In Climate in Europe | |
| AECM | Agri Environment Climate Measure | |
| AFN | Action For Nature |
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