This is when the federal government comes in.
If an area of law or regulation is not clearly assigned to either the provinces or the federal government, it may fall within the residual powers of the federal government. However, as new areas of the law emerge, deciding whether these new heads of power properly belong to either the federal or provincial government is often a difficult distinction to make. Governments have therefore relied regularly on the judiciary (the Courts) to make a final ruling.
In Canada, the courts are an independent judiciary and are expected, and able, to come to their own conclusions, despite what the other branches of government have done. It is this independence that has allowed the courts to play an important role as the decision maker when debates about the federal division of powers arise. The courts have often been responsible for ensuring that neither the federal nor the provincial governments are overstepping their role.  In making these decisions, the courts have had to rely on and interpret a number of constitutional principles.
For example, Section 92A (3) of the Constitution Act, 1982 states that provincial and federal regulation of natural resources can exist together so long as the provincial law does not directly interfere with the federal law’s main purpose. If and when that interference happens, the provincial law is considered invalid. 
The courts have also played a role in determining which activities fall under the residual federal power, found in the peace, order and good government clause in Section 91 of the Constitution.  This clause gives the federal government control over national emergencies or matters considered to have a “national dimension”. In particular, as new areas of environmental law have emerged, the governments and the courts have had to determine whether they can be controlled by the federal government under one of the overarching sections, such as peace, order and good government, or whether they should be allocated to the provinces.
Over time, unallocated powers such as control over aeronautics, radio, and official languages have gone to the federal government rather than to the provinces, under the peace, order and good government power. Further, through the lens of having “national dimensions”, certain matters – including those originally of a local nature and under provincial power (e.g., day-to-day health care) – can, through changed circumstances, such as an epidemic, become subject to federal jurisdiction. 
There are also other concurrent powers or areas of law that are exercisable by both jurisdictions – meaning that both the provincial and federal governments can make rules about the area of law in question. In those cases, the court may be called upon to distinguish whether a provincial law is intruding upon the main area of a federal law or whether it is a situation where both laws can exist side by side. In the event that both the provinces and the federal government enact laws covering the same or similar subject matters and there is a conflict between the two laws, the federal law stands. In turn, any parts of the provincial law that conflict with the federal one are not applicable. This is known as the doctrine of paramountcy.
One tool that courts use to determine which level of government has authority over a certain matter is known as the pith and substance test. The pith and substance test looks at the basic purpose and effect of a law to determine whether it falls within federal or provincial jurisdiction.
Some of these constitutional principles appeared front and centre in the Supreme Court hearing about the Greenhouse Gas Pollution Pricing Act.  This is the federal Act that puts a price on carbon – you may have heard it called a ‘carbon tax’. Three provinces, Alberta, Ontario, and Saskatchewan opposed the constitutionality of this Act, arguing that it falls within the provincial heads of power, specifically section 92(A) – provincial jurisdiction over natural resources.  All three provinces went to their respective Courts of Appeal where the decisions were split, the Ontario and Saskatchewan Court of Appeals both found the Act to be constitutional while the Alberta Court of Appeal disagreed finding that the Act illegally infringes upon provincial jurisdiction.  These cases were then appealed to the Supreme Court and the matter appeared in front of the Court in fall 2020.
The federal government is arguing that the Act falls properly under their constitutional power, specifically citing the residual jurisdiction over matters of “Peace, Order and Good Government” or section 91 of the Constitution. Specifically, they cite the national concern branch which was clarified in the case of R v Crown Zellerbach.  This case was about ocean dumping and the Supreme Court of Canada, in their decision, found that it was rightfully a matter of national concern falling under federal jurisdiction. 
In the Greenhouse Gas Pollution Pricing Act case, the federal government argued that the matter of national concern was “establishing minimum national standards that are integral to reducing Canada’s nationwide GHG emissions.” The provinces disagree, arguing that it is an unconstitutional intrusion into their powers. We will have to wait and see what the Supreme Court of Canada finds but, regardless of outcome, this will be a very important case for our interpretation of these complex Constitutional principles.
 Patrick J Monahan & Byron Shaw, Constitutional Law, 3d ed (Toronto: Irwin Law, 2013) at 10.
 Constitution Act, 1867, (UK), 30 & 31 Vict, c 3, reprinted in RSC 1985, App II, No 5, s 92(A)(3).
 Constitution Act, 1867, s 91.
 W.H. McConnell & Richard Foot, “Constitution Act, 1867” (30 January 2015) The Canadian Encyclopedia online: http://www.thecanadianencyclopedia.ca/en/article/constitution-act-1867/; Gerald A. Beaudoin & Jon Tattrie, “Constitutional Law” (6 July 2015) The Canadian Encyclopedia online: http://www.thecanadianencyclopedia.ca/en/article/constitutional-law/.
 Centre for Constitutional Studies, “Doctrine of Paramountcy” online: https://ualawccsprod.srv.ualberta.ca/2019/07/doctrine-of-paramountcy/.
 Centre for Constitutional Studies, “Pith and Substance” online: https://ualawccsprod.srv.ualberta.ca/2019/07/pith-and-substance/.
 Greenhouse Gas Pollution Pricing Act, SC 2018, c 12, s 186.
 Reference re Greenhouse Gas Pollution Pricing Act, 2020 ABCA 74 at para 25.
 Reference re Greenhouse Gas Pollution Pricing Act, 2019 ONCA 544; Reference re Greenhouse Gas Pollution Pricing Act, 2019 SKCA 40; Reference re Greenhouse Gas Pollution Pricing Act, 2020 ABCA 74.
 R v Crown Zellerbach,  1 SCR 401 [R v Crown Zellerbach]
 R v Crown Zellerbach, at para 40.
 Reference re Greenhouse Gas Pollution Pricing Act, 2020 ABCA 74 at para 27.
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